Methoxycarbonylation, stereoselective cyclization

SCHEME 15.5. Stereoselective cyclization using Pd(II)-catalyzed methoxycarbonylation. 

Cyclization of a variety of y-allenic alcohols with silver nitrate proceeds by 5-exo cyclization to form 2-alkenyltetrahydrofurans (equation 87).205c 206 Little stereoselectivity is seen in cyclizations of secondary alcohols. Cyclization by intramolecular oxypalladation/methoxycarbonylation or oxymercuration followed by transmetallation and methoxycarbonylation also showed no stereoselectivity (equation 88 and Table 24, entries 1 and 2).50 207 However, cyclization of the corresponding r-butyldimethylsilyl ether derivatives with mercury(II) trifluoroacetate followed by transmetallation/methoxycarbonylation pro-... 

Cyclization of 8-allenylamines is successful also. Synthesis of 2-alkenylpiperidines can be effected with mercury(II) or silver salts, with silver salts giving higher yields (equation 141 and Table 35).268a The significant asymmetric induction found in the cyclization of a chiral allene (78% ee, entry 2) suggests that the low stereoselectivity observed in the synthesis of the 2,6-disubstituted system (entry 3)269 may be a result of starting with a diastereomeric mixture. Aminopalladation/methoxycarbonylation has been effected in moderate yield also (entry 4). 

Unsaturated benzylidene and alkylidene acetals, under methoxycarbonylation conditions, give tetrahydro-2//-pyran-2-acetates in low to good yield. Moderate stereoselectivity is observed in the cyclization and the ra-2,6-disubstituted isomer is predominant in the reaction mixture. The preferred 2,6-cis relationship can be ascribed to the 2,6-diequatorial orientation of the substituents on the tetrahydro-2//-pyran ring83. 

---