Procedure: Hexamethyldisilazane (25g) is added dropwise to a stirred solution of 1,2,3,5-tetramethyl-4,6-di~hloroborazine[~~ (1 2.8g) in dry toluene with exclusion of moisture. After refluxing for 3 h and removal of toluene by distillation the residue is refluxed with a further portion (100 ml) of hexamethyldisilaZane for another 2.5h. The colorless residue obtained on distillation of the hexamethyldisilazane is subjected to molecular distillation torr) and yields a small amount of (2) at 120°C bath temperature and ( 3 ) at 150°C. O n heating to more that 500°C in an air bath compound ( I ) sublimes over. >=GG?44 trans: ( 4 I lUO% ( 9 ) \ + % (10) Received: January 16, 1975 [Z 183 IE] German version: Angew. Chem. 87, 382 (1975) CAS Registry numbers: ( I ) . 54750-92-4: (2). 54750-93-5: (3). 54750-94-6; 184.108.40.206-tetramethyl-4,6-dichloroborazine.7387-21-5 : hexamethyldisilazane. 999-97-3 [ l ] Cf. H . Steinherg and R . J. Brotherton: Organoboron Chemistry, Vol. 2. Wiley-Interscience, New York 1966.  M. F . Lappert and M . K . Majumdar, Proc. Chem. SOC. London 1963. 88. 131 A. Stock and E. Pohland. Chem. Ber. 59,2215 (1926).  H . S. Turner and R. J. Warne, Proc. Chem. SOC.London 1962.69. [ 5 ] A. Meller and M . Wechsherg, Monatsh. Chem. 98, 513 (1967).  R. J. Wagner and J . C. Bradford. Inorg. Chem. 1.93 (1962); R. H . Tornniskoetter and F . R . Half, ibid. 2. 29 (1963). bly attributable to direct opening of the C2-C3 bond of ( 5 ) with subsequent allenylmethyl-butadienyl rearrangement and [1,4]-hydride shift["]. Remarkably, it is always the carbon atom displaying the lowest degree of substitution (C3)which undergoes alkyl shift and 1,3-elimination in all the cited isomerizations. This leads to clear-cut reaction courses in all cases. [1,2]-AIkyl Isomerizationsof AlkenylidenecyclopropanesCatalyzed by Lewis Acids , (312% By Lutz Fitjer"] Cyclopropyl and cyclopropylmetbyl cations are usually generated in the presence of a nucleophile which is incorporated into the final product[''. However, they should also be accessible in the form of the complexes ( I ) and (2) in the absence of nucleophiles by reaction of suitably substituted methylenecyclopropanes with Lewis acids and thus permit pure isomerizations during cyclopropyl-ally1 or cyclopropylmethyl-cyclobutyl rearrangements. This is indeed the case. p@ (2) L60 Treatment of 2-methyl(2-methyl-l -propenylidene)cyclopropane ( 3 ) [ and trans-2,3-dirnethyl(2-methyl-l -propenylidene)cyclopropane (4)['l with boiling ethereal zinc iodide solution['] affords 3-isopropylidene- 1-methyl-1 -cyclobutene (8)[61 and 3-isopropylidene-1 ,4-dimethyl-l-cyclobutene(9)I3I, respectively, each as the sole reaction product. In contrast, 2,2dimethyl(2-methyl-1-propenylidene)cyclopropane (6)IZ1 and 2,2,3-trimethy1(2-methyl-1-propenylidene)cyclopropane ( 7)"' underwent quantitative isomerization to diisopropylidenecyclopropane ( I / ) ['I and 1,2-diisopropylidene-3-methylcyclopropane (/2)['l. cis-2,3-Dimethyl(2-methyl-l-propenylidene)cyclopropane (5)IZ1gave a mixture of ( 9 ) and 3-ethylidene-5methyl-l,4-hexadiene (/0)[31. As shown for the case of ( 3 ) , the isomerizations of (3)-(5) can only be explained in terms of complex formation via C' and subsequent [1,2]-alkyl and [1,2]-hydride In contrast, the isomerizations of ( 6 ) and (7) must be preceded by complex formation via C". In this case cyclopropyl-ally1 rearrangement and trans-1,3-elimination lead to formation of ( 2 1 ) and ( I 2 ) , respectively. Formation of (10) is presuma[*I Dr. L. Fitjer Organisch-Chemisches Institut der Universitat 34 Gottingen, Tammannstrasse 2 (Germany) 360 The above examples demonstrate that cyclopropyl, cyclopropylmethyl (or 1-cyclopropylvinyl), and possibly also allenylmethyl cations are accessible by complex formation of suitably substituted alkenylidenecyclopropaneswith Lewis acids, the nature of the cations formed being crucially dependent upon the substitution pattern of the cyclopropane ring. Received: January 15, 1975 [Z 187 IE] German version: Angew. Chem. 87, 381 (1975) CAS Registry numbers: (31, 54724-63-9; (41, 37817-46-2; (5). 37817-36-0; (6). 28438-32-6; 171. 14803-30-6; (81. 10412-56-3: (91, 54677-56-4: 1/01, 54677-57-5; 3761-85-1 ; 1/21. 13303-32-7: Znl,. 10139-47-6 ~ C. D. Gutsche and D. Redmore, Advan. Alicyclic Chem. Suppl. 1, p. 1 ( 1 968). (31 C31, (41 [4a], (5) [la]. f61 [4b, 4c], and (7) [4c, 4d] were prepared according to Crandall [4d] by reaction of 1-bromo-3-methyl1,2-butadiene with potassium tert-butoxide in an excess of the corresponding alkenes. The new compounds ( 3 ) . (9). and ( 1 0 ) gave correct analytical values; IR. UV, MS data accord with the structures given. The configuration of (10) was not determined. 'H-NMR data (100MHq CCI,, TMS int.): (3): 6=0.93 (m. LH), 1.16 (d, CH3, J = 6 H z ) , 1.47 (m, 2H), 1.68 (s, 2 C H 3 ) ; (9): 6=1.07 (d, CHI, J = 7 H z ) , 1.55 (br. s, 2 C H d 1.77(br. s. CH,), 2.88 (br. q. 1 H, J = 7 Hz). 5.95 (9, =CH--, J = 1.5 Hz); (10): 6=1.47 (d. CH,, J = 1 . 5 Hz), 1.59 (d, CH,. J = 7 . 5 Hz), 1.81 (d, CH,, J = 2.0 Hz), 4.80 (br. d, 1 H, J = 10 Hz), 4.89 (br. d, 1 H, J = 17 Hz), 5.50 (br. q, l H , J=7.5Hz), 5.53 (br. s, lH), 6.20 (dd, l H , J = 1 0 and 17 Hz). a) H. D. Hartzler, J. Amer. Chem. SOC. 83, 4997 (1961); b) ibid. 83, 4990 (1961); c) G. Leandri and C. Santelfi-Rourirr. Bull. SOC. Chim. Angew. Chem. internat. Edit. 1 Vol. 14 ( 1 9 7 5 ) 1 No. 5      [lo] Fr. 1970, 1515; d ) D. R. Paulson, J . K. Crandall. and C. A. Bunell. J. Org. Chem. 35, 3708 (1970). Isomerization was performed by refluxing solutions of ( 3 j - ( 7 ) (5mmol) in 2.0M (12.51111) ethereal zinc iodide solution for 2-4 h. The isomerizations can also be carried out on a preparative scale: l 0 m l of 3.5 M ethereal zinc iodide solution isomerized u p t o 70mmol (6). (8) was hitherto only accessible by quaternization of l-aminomethyl-3isopropylidenecyclobutane and subsequent Hofmann degradation (J. K. Williams and W H. Sharkey, J. Amer. Chem. SOC.81, 4269 (1959)). (11 j can also be prepared by reaction of l,l-dibromo-2-methyl-l-propene with methyllithium in the presence of 3-methyl-1.2-butadiene (R. F. Bleiholder and H. Shechrer, J. Amer. Chem. SOC. 86, 5032 (1964)). ( 1 2 j is also obtained alongside isomeric dialkylidenecyclopropanes on thermal isomerization of ( 7 ) [4d]. Compare the results obtained on solvolysis of substituted l-cyclopropylvinyl cations (D.R. Kelsey and R. G. Bergmann, J.C.S. Chem. Comm. 1973. 589). Alternatively, consecutive [1,3]- and [1,2]-hydride shifts are also conceivable; specific deuteration of ( 5 ) should clarify this matter. Facile Synthesis of ~-3(3,4Dihydroxyphenyl)alanine ( L - D o ~and ) Related Compounds By E.-0. Renth"] Dedicated to Professor Karl Zeile on the occasion of his 70th birthday Use of L-dopa ( 3 b ) in the treatment of Parkinson's disease prompted us to search for a simple and inexpensive synthesis of this compound. The synthesized L-dopa must be free from the D-enantiomer, which leads to clinical side effects. Our process consists in enzyme-induced (e.g. by papain) asymmetric synthesis. Substituted DL-N-benzoylphenylalanine derivatives are transformed into the optically active L-amino acid anilides ( 2 ) , which are then hydrolyzed to give the L-amino acids ( 3 ) . solution of about 3 g of papain in 0.05 N citrate buffer (300 ml) is then added and the pH adjusted to 5.80 by addition of glacial acetic acid or 1 N sodium hydroxide solution. After a short time the ~-anilide( 2 a ) begins to precipitate; after 48 h it is filtered off under suction, washed with water (300ml), and dried; m. p. 208-209 "C. ~-3-(3,4-Dihydroxyphenyl)alamne(L-dopa) ( 3 b ) L- N-Benzoyl-3-(3-hydroxy-4-methoxyphenyl)alany~ani~ide ( 2 c ) (9.75g, 25 mmol) is refluxed for 7 h with 48% hydrobromic acid (50ml). After cooling, benzoic acid is filtered off and rinsed with a small volume of cold 48% hydrobromic acid, and the slightly yellow solution evaporated down in uacuo. The residue is dissolved in methanol (50ml) and propylene oxide (5ml) is added. The temperature is maintained at 50°C until the pH value of 6.0 has been attained, and the mixture is then cooled and the product ( 3 6 ) is filtered off under suction and recrystallized from water. ~-N-Benzoyl-3-(4-hydroxy-3-methoxyphenyl)alanine (3a) and -3-(3-hydroxy-4-methoxyphenyl)alanine( 3 c ) The L-anilide ( 2 a ) or ( 2 c ) is refluxed for 16 h in the tenfold amount of 2~ HCI. The reaction mixture is then cooled and repeatedly extracted with ether. The aqueous phase is filtered over charcoal and vacuum distilled. The residue is dissolved in a fivefold amount of ethanol. Propylene oxide is added and the temperature maintained at 50°C for 1h until the pH value 6.0 is attained. The solution is allowed to stand for ca. 12 h in a refrigerator, filtered under suction, and (3a) or ( 3 c ) recr!\callized from water; the monohydrate is obtained. Received: February 6, 1975 [Z 199 IE] German version: Angew. Chem. 87.379 (1975) CAS Registry numbers: (10). 2901-78-2; ( 2 ~ ) 38152-95-3; . ( 2 c ) , 50713-71-8; ( 3 a ) , 300-48-1 ; (3b). 59-92-7; f 3 c ) , 35296-56-1 Preparation and Reactivity of Sterically Crowded Porphyrins'"] RID F- C F I Z - ~ : ~ COOH R2 R1 R2 L-12 J Yield [%I '[a]?,, ["I L-(3 I . Yield [%I in C H 3 0 H a OCH, OH b OH OH c OH OCH3 100 95 -!-40.0 c= 1 +41.8 ~ I [a]!& ["I in 1 N HCI 68 - 5.83 c= 1 69 -12.15 c=4 61 - 8.5 c=2 c = 0.5 ~-N-Benzoyl-3-(4-hydroxy-3-methoxypheny[e f2a) DL- N-Benzoyl-3-(4-hydroxy-3-methoxyphenyl)alanine (la) (31.5g, 0.1mol) is dissolved with warming in 0 . 0 5 ~sodium hydroxide solution (200ml). After addition of 2~ sodium acetate solution (150ml) the mixture is filtered. 0 . 0 5 ~citrate buffer (pH 5.0,300ml), cysteinium chloride (2g), and distilled aniline (18 ml) are then added successively to the filtrate. A [*] Dr. E.-0. Renth C. H. Boehringer Sohn, Abteilung Pharmachemie 6507 Ingelheim (Germany) By Ludger Witte and Jiirgen-Hinrich Fuhrhop"] Recent biochemical concepts on the activation of oxygen by heme enzymes". 21 and the photolysis of water by chlorophyll dimersL31should be amenable to checking with synthetic metalloporphyrins in uitro. Simple analogs of these important redox catalysts would be of interest from the preparative standpoint both for the oxygenation of olefins as well as for the hydroxylation of non-activated hydrocarbons. For the synthesis of rigid ligand fields about the central iron atom in heme analogs or of dimers having porphyrin planes at a well-defined distance, we require porphyrins containing the same side chains, which are capable of being coupled with each other, at oppositely lying methine bridges (u,y). From these it should be possible, e.g., to prepare stericaily fixed ansa porphyrins having rigid axial ligands through the central ion. To our knowledge, porphyrins with highly substituted methine bridges were previously only accessible by lengthy total synthesis14! We now wish to report on their prepara[*] Dip1.-Chem. L. Witte and Doz. Dr. J.-H. Fuhrhop Gesellschaft fur Molekularbiologische Forschung mbH 3301 Stockheim bei Braunschweig, Mascheroder Weg 1 and Institut f i r Organische Chemie A der Technischen Universitat Braunschweig (Germany) [**I This work was supported by the Bundesministerium f i r Forschung und Technologie as part of the technology program, by the Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie.