Migration phenylthio groups

Thermodynamic control of the ratio of 127 to 128 formed was shown by the conversions of 119,121,122, and 124, under identical conditions, into mixtures of the epimers in which no significant difference in the proportions could be discerned.20 The formation of 127 and 128 may be rationalized as proceeding by the following steps (a) acid-catalyzed hydrolysis of the 4,5-acetal group and loss of the 3-substituent from 119-124, (b) attack of 0-4 at C-l of the resulting allylic carbonium ion, (c) hydrolysis of one phenylthio group, (d) migration of the other to C-3 by an internal, Sn2 type of process, and (e) addition of water at C-l. 

The structure of the penam product denounces migration of a phenylthio group, which requires the involvement of dipolar species 320 in the C2-C3 bond formation. This intermediate would arise from the 3-thiacepham sulfonium ylide 319, which is a reasonable product of intramolecular carbene trapping (318 - 319) by the thioether sulfur atom, in the absence of the thioxo sulfur atom. Thus, three different 3-thiacepham dipolar species (313,314,319) formally participate in as many penem ring formation strategies. This is one of the fascinating aspects of P-lactam chemistry. 

The epoxide below reacts stereo- and regio-selectively by [, > -endo migration of a phenylthio group in almost neutral conditions (Scheme 60). ... 

Scheme 3.71 Alkyl migration including ring contraction from a migration origin bearing a phenylthio group.

An efficient process for one-carbon homologation to aldehydes is based on cyclic boronate esters.17 These can be prepared by hydroboration of an alkene with dibromobor-ane, followed by conversion of the dibromoborane to the cyclic ester. The homologation step is carried out by addition of methoxy(phenylthio)methyllithium to the boronate ester. The migration step is induced by mercuric ion. Use of enantioenriched boranes and boronates leads to products containing the groups of retained configuration.18... 

If the pKa of the corresponding acid R1 - H from the stabilized carbanion is smaller than 35, the migration of R1 fails in (dichloromethyl)borate complexes. Failure to convert pinanediol [(phenylthio)methyl]boronate to an a-chloro boronic ester has been reported15. Reaction of (dichloromethyl)lithium with an acetylenic boronic ester resulted in loss of the acetylenic group to form the (dichloromethyl)boronate, and various attempts to react (dichloromethyl)boronic esters with lithium enolates have failed17. Dissociation of the carbanion is suspected as the cause, but in most cases the products have not been rigorously identified. 

The corresponding photocyclization of S-arylvinyl sulfides to dihydro-thiophens has been shown to proceed via short-lived thiocarbonyl ylide intermediates, formed in turn by cyclization of triplet excited states. 1-Phenylthio-3,4-dihydronaphthalene (49), for example, affords the colored thiocarbonyl ylide (50)44 the isolated products are derived by processes involving hydrogen abstraction and migration. The greater efficiency of the photocyclization of 2-thioaryloxyenones (51) to dihydrothiophens (52) has been attributed to the additional stabilization afforded to the ylide (53) by the carbonyl group.45 Other similar photocyclizations have been reported.46... 

Reactions of Imidazoles. Thermolysis of 1-triphenylmethylimidazole results in migration of the imidazolyl group to yield compound (286). Sensitized photo-oxygenation of 4,5-diphenylimidazole in methanol affords a mixture of the imidazolinone (287) and the imidazolidinone (288). The imidazole (289 R = H) undergoes lithiation at C-5 subsequent treatment with diphenyl disulphide gives the di(phenylthio)-derivative (289 R = PhS). Copyrolysis of 2,4-dimethylimidazole and chloroform results in a complex mixture, containing imidazole and methyl-, dimethyl-, and chloromethyl-pyrimidines and -pyrazines. The confusion about the structures of N-methyl derivatives of iodo-nitro-imidazoles has been cleared up the supposed 1-methyl-2,5-di-iodoimidazole is actually the 4,5-di-iodo-compound it yields 4-iodo-l-methyl-5-nitroimidazole on nitration. The reaction of the bromo-... 

In the field of reactions of thioglycosides the photobromination of phenylthio compounds has been covered in a review. As has been noted before, and has been used usefully in synthesis of 2-deoxy compounds, the alkylthio group of thioglycosides may migrate to C-2 under certain conditions. Thus, as well as compound 93 giving the expected 2,3-orthoester on treatment with alkyl orthoesters in acid conditions, it gave compound 94... 

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