From Ylides

Disubstituted from ketone Monosubstituted from ylide... 

Oxaphospholans.—The stable phosphoranes (41) were obtained from ylides and epichlorohydrin.35 The formation of (41 R = H) from the action of base on the salt (42) supports a mechanism via the intermediate ylides (43). The crystalline cyclic... 

Reactions.—Aldehydes. The stereochemistry of the alkene produced from ylides generated by using 18-crown-6 complexes of potassium carbonate or butoxide, depends upon the solvent used. In THF typical salt free distributions are obtained whereas in dichloromethane reversal of product distributions is observed.17 A simplified method (Scheme 4) for preparing para-substituted styrenes in high... 

Miscellaneous. An interesting synthesis of 1,1-difluoro-l-alkenes from ylides and chlorodifluoromethane has been described.47 The ylide acts both as a carbene generator and trapping agent (Scheme 11). 

Fig. 4.1. Generation of acceptor-substituted carbene complexes from ylides. X N2, SR2, S(0)Me2, Arl Z COR, CO2R, CONR2, SO2R, CN, P(0)(0R)2.

Scheme 23 Different orthoplatination reactions from ylide and phosphonium salts...

Additional heterocyclic ring systems, such as benzofurans [125], dihydropyrroles and dihydroazepines [41], piperidines and dihydropyrimidines 36 [126], and fused oxazole derivatives [127], have been described (Eq. 7). The formation of epoxides and aziri-dines, formally emanating from ylides, was recently reported by Doyle et al. [77]. Rho-dium(II)-catalyzed isomiinchnone cycioaddition followed by Lewis acid-mediated ring opening has been used as an entry into the protoberberine azapolycyclic ring structure [128]. 

Mangalagiu studied the regioselectivity of the 1,3-dipolar cycloaddition of several pyridazinium methylides 105 to ethyl acrylate, ethyl propiolate, and acrylonitrile. The reaction is HOMO controlled from ylides and only one regioisomer 106 (major isomer as and minor isomer trans) or 107 is formed, namely the one in which the ylide carbanion makes a new bond with the most electrophilic carbon of the 1,3-dipolarophile. In some cases oxidation of 106 to 107 is observed in the reaction mixture in contact with the air (Scheme 23), which can be avoided by working in N2 atmosphere <1996T8853, 1997ACS927, 1999EJO3501>. 

Epoxide Formation from Ylides Prepared by Means of Bases... 

Germanyl- and stannyl-substituted ylides may be synthesized starting from ylides and germanyl and stannyl chlorides e.g. equation 46). Methylenetriphenylphosphorane is converted into the corresponding disubstituted ylide (equation 47). The monosubstituted derivatives are available by desilylation of 1-trimethylsilylmethylenephosphorane (c/. equation 36). 

Alkylidenephosphoranes which carry a carbamoyl or thiocarbamoyl group in the a-position are formed from ylides and isocyanates or thioisocyanates (equation 79).im,i73 jf starting ylide has no activated a-H atom the reaction with thioisocyanates stops at the betaine intermediate (see equation 78) which can subsequently be alkylated and transformed into the corresponding substituted ylide with an external base. 

Rearrangements via oxonium ylides are scarce. Nonetheless, interesting structures were prepared from ylides like (6 equation 3) or (7 equation 4). The potential of ylide rearrangement methodology in the context of total synthesis is unexplored, but interesting target molecules are already identified. ... 

The methylene transfer reaction from ylides to ketones has been developed as a convenient synthetic method for obtaining oxiranes [24]. However, the experimental procedure is complex. For example, a THF solution of dimethylsulfonium-methylide (61) is obtained by treatment of trimethylsulfonium iodide (60) with BuLi in THF at 0°C, and after addition of the ketone the mixture is heated at 50-55 °C under nitrogen to yield the oxirane. Throughout the reaction and separation of the product, the organic solvent is essential [25]. 

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