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The continued fascination chemists possess with asymmetric synthesis provides the basis for the next four procedures. The synthesis of (R)-(-)-10-METHYL-l(9)-OCTALONE-2 is a nice demonstration of an asymmetric Michael addition by a chiral imine followed by an aldolâ€”in short an asymmetric Robinson annulation. The asymmetric glycolization to STILBENE DIOL (R,R-l,2-DIPHENYL-l,2-ETHANEDIOL) represents an olefin oxidation using catalytic alkaloids in tandem with osmium tetroxide. As reagents for a variety of asymmetric alkylations, the preparation of 2-CYANO-6-PHENYLOXAZOLOPIPERIDINE is presented as well as another route to
(S)- AND (R)-l,l'-BI-2-NAPHTHOL, obtained by resolution of its pentan-oate ester with cholesterol esterase.
In accordance with the editorial boardâ€™s recent policy reflecting its interest in obtaining more procedures for important heterocyclic compounds, seven procedures for heterocyclic ring syntheses and three of substituent modification follow: 3,4-DIETHYLPYRROLE and the highly symmetric OCTA-ETHYLPORPHYRIN are described making this synthetic porphyrin readily available for biological modeling and inorganic chemical applications.
This is followed by an example of a pyrrole synthesis: DIMETHYL-3-PHENYLPYRROLE-2,5-DICARBOXYLATE is obtained via an inverse demand Diels-Alder synthesis using DIMETHYL-1,2,4,5-TETRAZINE-
3,6-DICARBOXYLATE. A rhodium-catalyzed cyclization of acetylenes with a-diazocarbonyls provides a nice route to ETHYL 2-METHYL-5-PHENYL-
3-FURAN CARBOXYLATE and related 2,3,5-trisubstituted furans. Two procedures dealing with cyclization to nitrogen heterocycles are includedâ€” the first is an example of iodolactamization to give 8-EXO-IODO-2-AZA-BICYCLO[3.3.0]OCTAN-3-ONE and the second procedure is a nice demonstration of an alkyne-iminium cyclization furnishing (E)-l-BENZYL-3-(l-IODOETHYLIDENE)PIPERIDINE. A sulfur heterocycle, 9-THIABICY-CLO[3.3.1]NONANE-2,6-DIONE is also included which has shown to be a versatile intermediate to a number of other sulfur heterocycles. The current interest in cryptands for complexing metal ions provides the rationale for the procedure describing (TRIAZA-21-CROWN-7)4-BENZYL-10,19-DI-ETHYL-4,10,19-TRIAZ A-1,7,13,16-TETRAOXAC YCLOHENEICOS-ANE.
Modification and elaboration of heterocycles are demonstrated by the transformation of dihydroisoquinolines to 7,8-DIMETHOXY-l,3,4,5-TETRA-HYDRO 2H,3-BENZAZEPIN-2-ONE, the conversion of 2-bromopyrrole to N-tert-BUTOXY-2-TRIMETHYLSILYLPYRROLE, and substitution of the a-phenylsulfonyl group in pyran to furnish TETRAHYDRO-2-(PHEN-YLETHYNYL)-2H-PYRAN.
The next group of procedures appearing herein reflects a variety of useful new reagents for performing a variety of synthetic tasks. The preparation of TRIS(TRIMETHYLSILYL)SILANE as a hydrogen donor source in free radical chemistry is a welcome addition, as is the preparation of 9-BORA-BICYCLO[3.3.1]NONANE DIMER, the well recognized hydroboration agent. A stable methylene transfer agent, IRON (1+), DICARBON-YL(ri s-2,4-CYCLOPENTADIEN-l-YL) (DIMETHYLSULFONIUMii-METHYLIDE)-,TETRAFLUOROBORATE(l ) is described and shows its utility in the synthesis of 1,1-DIPHENYLCYCLOPROPANE. The clcan
1.2-addition of RCu(CN)ZnI reagents to a,/3-unsaturated aldehydes is nicely demonstrated as well as the 1,4-addition of ORGANOBIS(CUPRATES) in a spiroannelation affording 9,9-DIMETHYLSPIRO[4.5]DECAN-7-ONE. The interesting properties of ALKYNYL ARYL IODONIUM SALTS are highlighted in the preparation of the conjugated enyne, E-5-PHENYL-DO-DEC-5-EN-7-YNE.
The last group of procedures included in this volume represent efficient means of reaching important and pivotal intermediates for a host of function-alized materials. The simple, single step, synthesis of 2-METHYL-l,3-CY-CLOPENTANEDIONE from cheap materials is presented, followed by a convenient procedure leading to large scale preparation of the functionalized diene, (E,Z)-l-METHOXY-2-METHYL-3-(TRIMETHYLSILYLOXY)-
1.3-PENTADIENE. Another useful diene, generated under nickel catalysis between a 1,3-dithiolane and a Grignard reagent, is (E,E)-TRIMETHYL(4-PHENYL-1,3-BUTADIENTYL)SILANE. A rapid entry into fluorine-containing alkynes is the procedure describing the pyrolysis of a-ACYL-METHYLENEPHOSPHORANES which produces a good yield of ETHYL
4,4,4-TRIFLUORO-2-BUTYNOATE. C-Acylation of an enolate using methyl cyanoformate provides a convenient source of the a-carbometh-oxyoctalone, METHYL (la,4Ap,8Aa)-2-OXO-DECAHYDRO-l-NAPH-THOATE and represents a good example of generating /3-keto esters under mild conditions. The nitrone functionality is featured in a procedure which makes it in a single step from secondary amines and 6-METHYL-2,3,4,5-TETRAHYDROPYRIDINE N-OXIDE is the example described. Finally, the synthesis of phospholes: 1-PHENYL-2,3,4,5-TETRAMETHYLPHOS-PHOLE is described as an example of the versatility of zirconocene chemistry.
In closing, the editor acknowledges the skills, patience, and dedication of all my colleagues on the Editorial Board of Organic Syntheses and their students and coworkers who painstakingly checked all the procedures included in this edition. Furthermore, the guidance, calming effect, and talent of Professor Jeremiah Freeman, Secretary to the Board, in demanding and maintaining the highest standards are truly appreciated. The final, clearly discem-able, and attractive product before the reader is due entirely to Dr. Theodora W. Greene, Assistant Editor to Organic Syntheses, and the Freeman staff at the University of Notre Dame who typed and prepared the final manuscript.