Grignard reagents 3,3 -sigmatropic rearrangements

Conjugate addition of vinyllithium or a vinyl Grignard reagent to enones and subsequent oxidation afford the 1.4-diketone 16[25]. 4-Oxopentanals are synthesized from allylic alcohols by [3,3]sigmatropic rearrangement of their vinyl ethers and subsequent oxidation of the terminal double bond. Dihydrojasmone (18) was synthesized from allyl 2-octenyl ether (17) based on Claisen rearrangement and oxidation[25] (page 26). 

The thiophilic addition of Grignard reagents was also carried out with S-allylic and propargylic dithioates. In these cases a [2, 3] sigmatropic rearrangement of the magnesiocarbanion occurred, and alkylation at sulfur yielded the dithioacetals of (i-unsaturated ketones [334] or of allenic ketones [335]. 

This facile type of sigmatropic of rearrangement appears general. Julia and his group [96-98] have prepared and characterised a number of unsaturated sulfines. Instead of an oxidation route to the starting sulfoxides, they performed the reaction of vinyl Grignard reagents with allyl sulfenates at -78 °C. In most cases the subsequent [3.3] shift is so rapid that the intermediate unsaturated sulfoxides were not even evidenced sulfines were isolated instead. 

Another useful reaction of these Diels-Alder adducts is shown in equation (54). Dihydrothiazine oxide (123) from ( . )-2,4-hexadiene can be opened with a Grignard reagent to allylic sulfoxide (124) which undergoes a stereoselective 2,3-sigmatropic rearrangement via the envelope-like transition state conformation shown, having a quasi-equatorial methyl group to afford sulfenate ester (125). Desulfurization of (125) provides E)-threo smino alcohol derivative (126) in excellent overall yield. If ( ,Z)-2,4-hexadiene is used, the E)-erythro epimer of (126) is formed cleanly. 

Displacement-rearrangement. The reaction of allenesulfinate esters with alkenyl Grignard reagents proceeds by displacement on sulfur. [2,3]Sigmatropic rearrangement and dimerization follow. 

Sinensal (18) and /3-farnesene (19) have both been synthesized from the thioncarbamate (16), which undergoes a [3,3] sigmatropic rearrangement to produce the allylic thiolcarbamate (17) (Scheme 1)/ The Grignard reagent... 

---