Sulfoxides, allenyl 2,3 -rearrangements

Vinyl sulfoxides have been used as synthetic equivalents of alkynes in reactions with diazoalkanes to prepare pyrazoles. The initially obtained adducts subsequently eliminate or rearrange the sulfoxide moiety to achieve pyrazoles lacking the sulfur function. Thus, the adducts resulting by reaction of CH2N2 with the sulfinylated double bond of allenyl sulfoxides 213 are transformed through a sulfoxide-sulfenate rearrangement into hydroxymethyl pyrazoles 214 [168], whereas those obtained by reaction with sulfinyl coumarins 215 suffered sulfinyl elimination into the pyrazoles 216 [169]. In both cases l,H-pyrazoles were obtained as a consequence of a final tautomerization step (Scheme 101). These studies were carried out on racemic sulfoxides. 

The role of allylic sulfoxides as homoaldol equivalents in the synthesis of ( )-allylic alcohols was summarized earlier. A more recent finding is that allylic sulfoxides are precursors for conjugated dienes (equation 70). The elimination is regioselective but stereorandom. The 2,3-rearrangement of propar-gylic sulfenates gives allenyl sulfoxides. Allenyl sulfoxides are valuable synthetic intermediates. They can be converted into stereochemically homogeneous allenes, e.g. (205 equation 71). ... 

In one of its earliest applications, Horner and Binder143 prepared a variety of allenyl aryl sulfoxides by the rearrangement of propargylic arenesulfenates, and noticed that accumulation of electron-withdrawing groups in the aromatic ring retard the rate of rearrangement. These authors have also performed a detailed study of the chemistry of... 

The synthetic utility of the remarkably facile and efficient [2,3]-sigmatropic rearrangement of propargylic sulfenates has been further demonstrated in a variety of preparations and interesting reactions of allenyl sulfoxides , including the preparation of vinylallenes " which are useful intermediates in organic synthesis in general and natural polyenes, such as Vitamins A and D, in particular Two typical examples, taken... 

Nevertheless, new examples have also been reported of allenyl sulfoxides [101] and especially allenyl trichloromethyl sulfoxides [39, Y+-X = S(0)CC13] [102]. There have been many trials to use these sigmatropic rearrangements for the synthesis of diallenes (compare 40 — 42), e.g. the reaction of the diols 43 with trichloromethane-sulfenyl chloride 44 to the diallenes 45 (Scheme 7.7) [103]. 

Cutting and Parsons described the transformation of acetylenic alcohols 314 into allenyl phenyl thioethers 316 by a two-step procedure (Scheme 8.85) [174], Deprotonation of alkynes 314 with n-butyllithium is followed by addition of phenylsulfenyl chloride, forming sulfenyloxy intermediates which subsequently rearrange to allenic sulfoxides 315. Treatment of allenes 315 with methyllithium results in loss of the sulfoxide moiety to form allenyl sulfides 316 in reasonable yields. 

The cycloaddition of allenyl sulfoxide 135 and cydopentadiene occurred at room temperature, giving the single adduct 136. The initially formed allylic sulfoxide underwent a rapid [2,3]-sigmatropic rearrangement. Treatment of 136 with trimethyl phosphite furnished alcohol 137. It should be noted that the reaction of methyl 4-hydroxy-2-butynoate with cydopentadiene failed to give 137. Thus, the allene 135 is considered as a masked and more reactive alkyne equivalent. 

In the case of steroidal propargylic alcohols the first rearrangement produced a mixture of allenyl sulfoxides, epimeric at the sulfur atom, which reacted with an added nucleophile to produce substituted allylic sulfoxides. Rearrangement of the sulfoxide resulted in the exclusive formation of a-hydroxy derivatives. This reaction sequence has been applied in a synthesis of hydrocortisone acetate74 (Nu = OCH3) from androstene-3,17-dione and in a transformation of mesantrol75 (Nu = malonate) to a spirolactone. 

The first [2,3] sigmatropic rearrangement of a propargylic sulfenatc to an allenyl sulfoxide 14 was observed during the attempted preparation of l-phenyl-2-propynyl trichloromethanesulfe-nate (13) by reaction of trichloromethanesulfenyl chloride with l-phenyl-2-propynol108. 

The addition of an allyl alcohol to racemic allenyl sulfoxides results in vinyl ethers with the sulfinyl moiety at C-1 that undergo sigmatropic rearrangements upon distillation to produce 2,4-dienones after ehmination of sulfenic acid. In one example, an isomeric vinyl ether was obtained with a sulfinyl methyl substituent at C-2 that gave rise to a sulfinyl enone upon rearrangement [138]. In related work, the addition-elimination of an allyl alkoxide to a functionalized vinyl sulfoxide results in a sulfinyl enol ether that rearranges with loss of sulfenic acid to the unsaturated ester [139-141] (Scheme 21). 

In general, synthesis of 4-isoxazolines is accomplished via three routes 1,3-dipolar cycloaddition of nitrones to alkynes or by the oximation of a,i -ethylenic ketones, a-alkynic ketones and aldehydes or from the selective reduction of isoxazolium salts. The nitrone (262) underwent tandem cyclo-addition-[2,3]sigmatropic rearrangement with allenyl sulfoxide (263). And it resulted in 4-isoxazoline (264) (Equation (46)) <89TL663>. 

Indolylmethyl phenyl sulfoxides. a-Ethynyl-o-acetamidobenzyl alcohols undergo 0-sulfenylation, which is immediately followed by a [2,3]sigmatropic rearrangement. The ensuing allenyl sulfoxides are prone to cyclize. 

DienonesJ A new synthesis of 2,4-dienones (4) involves reaction of an allylic alcohol (1) with an allenyl phenyl sulfoxide (2) in benzene (ice bath). The resulting enol ether (3) is treated with this base, which induces Claisen rearrangement and elimination of benzenesulfenic acid, with formation of the dienone 4 (equation I). 

Scheme 18.71 Rearrangement of iodoso alkene in synthesis of quinic acid derivative.— Scheme 18.72 Propargyl and allenyl sulfenate and sulfoxide f2.31 rearrangements.—...

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