Elimination reactions regiospecificity

The synthesis of vinyl azides by regiospecific elimination of hydrogen iodide from vicinal iodoazides has been developed by Hassner and co-workers and constitutes the only noteworthy example of the directing effect of the azido group on elimination reactions. 

Jin, Z., Kim, S.H. and Fuchs, PL. (1996) Phosphazene base P2-Et mediated isomerization of vinyl sulfones to allyl sulfones. Tetrahedron Letters, 37, 5247-5248 Jin, Z. and Fuchs, PL. (1996) Use of phosphazene base and phase-transfer conditions for regiospecific alkylative isomerization of vinyl capable of undergoing P-elimination reactions. Tetrahedron Letters, 37, 5249-5252 Kim, S.H., Figueroa, 1. and Fuchs, P.L. (1997) Application of the Grubbs ringclosing metathesis for the construction of a macrocyclic ansa-bridge. Synthesis of the tricyclic core of roseophilin. Tetrahedron Letters, 38, 2601-2604. 

The elimination occurs regiospecifically away from the hydroxyl group and the reaction occurs under mild conditions. 

Ailyl enol carbonates derived from ketones and aldehydes undergo Pd-cat-alyzed decarboxylation-elimination, and are used for the preparation of a, /3-unsaturated ketones and aldehydes. The reaction is regiospecific. The regio-isomenc enol carbonates 724 and 726, prepared from 723, are converted into two isomeric enones, 725 and 727. selectively. The saturated aldehyde 728 can be converted into the a,/3-unsaturated aldehyde 730 via the enol carbonate 729[459]. 

The allylic cation (40), formed in a specific acid-catalysed process, is relatively stable thermodynamically, stable enough towards trapping by nucleophiles that the reaction product obtained is almost invariably the naphthalene elimination product. di-Enediynes (42) are formed regiospecifically when the allylic cation (41) is trapped as shown. The walking of methanol around optically active l-methyl-3-ethylallyl... 

Diazo-substituted 1,2,3-triazoles undergo regiospecific dipolar cycloaddition reactions with electron-rich unsaturated compounds. Thus, 4-diazo-5-phenyl-4//-1,2,3-triazole (272, R = Ph) reacts with 1 -morpholinyl-2-nitroethene (273) in CH2CI2 at 20 °C by a net 1,7-cycloaddition and elimination of morpholine to give the product (274) (Scheme 53). 4-Diazo-5-cyano-4/f-1,2,3-triazole (272, R = CN) similarly adds to phenylacetylene to form compound (275) <87JOC5538>. 

Regiospecific 1,3-dipolar cycloaddition reactions of dihydropyridines and some organic azides lead to high yields of fused aziridines—2,7-diazabicy-cli[4.1.0]hept-3-enes 337 [365, 366, 367] (Scheme 3.112). The reaction proceeds via the preliminary formation of an intermediate 336 and the elimination of nitrogen. Reaction of pyrimidine 334 with less reactive methoxycarbonyl and benzoyl azides does not occur [367]. Compounds 337 exhibit significant analgesic, antibacterial and antifungal activities [367]. 

The synthesis of alkali metal 1,4,2-diphosphastibolides parallels that of the 1,4,2-diphosphaarsolides 18 and 19. It is however regiospecific and no 1,2,4-isomer is formed. For the synthesis, a DME solution of lithium bis(trimethylsi-lyl)antimonide 31 (M = Li) is treated with 3equiv of the phosphaalkene 29. In the course of the reaction, the phosphaalkene 29 is converted to the phosphaalkyne 30 via the base-catalyzed elimination of hexamethyl disiloxane (Scheme 7). Alternatively, the phosphaalkyne 30 can be used directly in place of the phosphaalkene. After addition of TMEDA or 12-crown-4, the lithium 1,4,2-diphosphastibolide 22 (M = Li(TMEDA)2) or Li(12-crown-4)2 is isolated <1997JOM291>. 

Diazo-substituted 1,2,3-triazoles undergo regiospecific dipolar cycloaddition reactions with electron-rich unsaturated compounds. Thus, 4-diazo-5-phenyl-4//-l,2,3-triazole 410 reacts with l-morpholinyl-2-nitroethene by a net 1,7-cycloaddition and elimination of morpholine to give the product 411. 

Jin and Fuchs reported that vinyl sulfones, using basic phase-transfer catalyst conditions, were regiospecifically alkylated at the a-position.78 No P-elimination products were observed in systems capable of undergoing anion-promoted P-elimi-nation. He also reported that y-methoxy vinyl sulfones 116 can be converted to the corresponding substituted enones 119 using this protocol (Scheme 32).79 On reaction with tert-butyllithium, 116 is converted to the y-methoxy allylsulfonyl anion 117, which was regiospecifically trapped by a variety of electrophiles to provide the enol ether 118. On hydrolysis the -substituted enone 119 was obtained. 

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