Sulfoxides, allenic synthesis

The male bean weevil sex attractant has been made by a new method of allene synthesis from aldehydes and alkenyl aryl sulfoxides by sulfoxide-metal exchange as the key reaction. 

The allene synthesis strategy outlined in equation 21 can be rendered asymmetric if non-racemic sulfoxides such as 47a are employed26. The diastereoselective step is an equilibration of the sp3-allylic organometallic species 53 before the -elimination step (equation 23). 

Arai Y., Koizumi T. Synthesis and Asymmetric Diels-Alder Reactions of Chiral. Alpha.,.Beta.-Unsaturated Sulfoxides Bearing a 2-Exo-Hydroxy-lO-Bornyl Group As an Efficient Ligand on the Sulfur Center Rev. Heteroat. Chem. 1992 6 202-217 Keywords allenic sulfoxide, a-sulfinylmaleate, a-sulfinylmaleimide, asymmetric synthesis, chiral unsaturated sulfoxides... 

Sulfur-containing allenes can be subdivided into donor-functionalized allenes such as allenyl thioethers and into acceptor-functionalized allenes such as allenyl sulfoxides and sulfones. In this section only the synthesis and chemistry of donor-substituted sulfur-containing allenes will be summarized. 

In a similar manner, Brummond et al. demonstrated the first total synthesis of 15-deoxy-A12,14-prostaglandin J2 (162) that was completed using a silicon-tethered allenic Pauson-Khand reaction to obtain the highly unsaturated cyclopentenone substructure [36]. Treatment of alkynylallene 160 with molybdenum hexacarbonyl and dimethyl sulfoxide affords the desired cycloadduct 161 in 43% yield (Scheme 19.30). Trienone 161 was obtained as a 2 1 Z E mixture of isomers in which the Z-isomer could be isomerized to the desired E-isomer. The silicon tether was cleaved and the resulting product converted to 15-deoxy-A12,14-prostaglandin J2 (162). 

The sulfenate-sulfoxide and sulfinate-sulfone rearrangements are very reliable and proceed with complete syn stereoselectivity17, ls. The allenic sulfoxides can be used for the synthesis of chiral alkylallenes with retention of configuration (see Section 1.1.3.). The relative configuration at sulfur in the allenic sulfoxides is not important for further synthetic purposes and racemization at sulfur is often observed without affecting the allenic axial dissymmetry. 

Yield of allenes 47a-e from sulfides unbranched in a-position to sulfur are moderate to high. However, a-branching to sulfur lead to lower yields (allenes 47f-h) and is accompanied by competing formation of sulfoxides by 0-transfer from oxaziridine 44. The metal-free reaction conditions allow synthesis of particularly sensitive allenes or those derived from sensitive sulfides... 

Scheme 23 /S-Allenic sulfoxides in the one-pot synthesis of functionalized 4-oxo-2-alkenoate esters...

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). ... 

This sulfoxide-magnesium exchange reaction could be successfully applied to a new synthesis of allenes (Scheme 3.12) [6]. The procedure is a novel method for synthesis of allenes from three components, ketones, chloromethyl p-tolyl sulfoxide, and sulfones, in relatively short steps. A key step is an attack of the lithium a-sulfonyl carbanion on the electron-deficient carbene carbon. /1-Elimination of the sulfoxyl group then occurs to give the allene. 

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