Sulfoxide, adducts with Lewis acid

Sulfoxides form adducts with Lewis acids like SbCls and SnCU [80]. In the case of SsO the crystalline products SsO-SbCh [81] and (SsO)2-SnCl4 [82] have been prepared and characterized by single crystal X-ray diffraction analyses. Reaction of SsO with SbCls in CS2 at 20 °C and subsequent cooling of the solution to -50 °C resulted in orange orthorhombic crystals of SsO-SbCls in 71% yield. These molecules are of Cg symmetry see Fig.6 [81]. 

Reactions of compound 8a with butadiene, 2,3-dimethylbutadiene, and isoprene required more drastic conditions (150°C, autoclave) [24]. In the case of isoprene, a 60 40 regioisomeric mixture was obtained (the major product being the para-adduct). The influence of Lewis acid catalysis on the reaction of cyclopentadiene with different ethynyl aryl sulfoxides [Ar =p-Tolyl, o-nitrophenyl, and l(2-methoxynaphthyl)] has been recently reported [25].Dienophilicities of acetylenic sulfoxides were greatly enhanced by the presence of various Lewis acids (the effect being greater with stronger Lewis acid), but the stereoselectivity was scarcely affected by the catalysis. 

C2-symmetric cyclic alkenyl sulfoxide (187), with cyclopentadiene, proceeded under mild conditions to give a single diastereomeric adduct (188) in excellent yield (Scheme 48). Other acyclic dienes also gave single diastereomeric adducts often without the necessity of using Lewis acids. The Ws-sulfoxide moiety can be readily deprotected using a two-step sequence of sulfoxide reduction followed by hydrolysis of the dithiolane to give the enantiomerically pure norbomenone (189). 

Asymmetric ene reaction of N-sulfinylcarbamatesf The ability of Lewis acids to promote ene reactions (11,413,414 12,389) is useful for asymmetric reactions. Thus the SnCU-promoted reaction of chiral N-sulfinylcarbamates (1) with alkenes results in thermally unstable adducts (2) in 65-91% yield. Use of trans-2-phenylcyclohexanol (13,244) or 8-phenylmenthol as the source of chirality results in high diastereoselective induction in generation of the new carbon to sulfur bond (usually >95 5). This reaction is applicable to both (E)- and (Z)-alkenes, but the former react more readily. These ene adducts can be transformed into optically active allylic alcohols (4) by N-alkylation and conversion to an aryl allylic sulfoxide (3), which undergoes rearrangement in the presence of a thiophile (piperidine) to 4, with retention of configuration at carbon imparted in the ene reaction. The overall process effects enantioselective allylic oxidation of an alkene with retention of the original position of the double bond. 

Ene Reactions. In the presence of Lewis acids, isobutene adds to various enophiles to yield ene adducts. Enophiles such as alkoxyaldehyde (eq 13), dialkyl aminoaldehyde (eq 14), haloaldehyde (eq 15), and vinyl sulfoxides (eq 16) have been utilized. Chiral organoaluminum (eq 17) and organotitanium reagents (eq 18) have been reported to give high levels of asymmetric induction in the ene reaction of isobutene with activated aldehydes. 

---