Allenes mercury salts

When treated with catalytic amounts of acid,404 403 silver salts406"410 or mercury salts,409,410 allenic alcohols will cyclize to ethers (equations 230-253). By proper choice of reaction conditions, the yields of products obtained in these cyclizations are generally high. Interestingly, the stereoselectivity observed in equation (253) is proposed to arise from a chair conformation of the intermediate.408... 

Allenic alcohols with a variety of allene substitution patterns form 2-alkenyltetrahydropyrans by a 6-exo ring closure upon cyclization with silver or mercury salts (equation 91 ).209 The organosilver intermediate undergoes protiodemetallation under the reaction conditions, while the vinylmercury... 

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). 

A similar electrophilic activation of the vinylallene to 2-cyclopentenone cyclization is possible using either mercury(II) acetate or thallium acetate. Solvometallation of allenes is a well studied pro-cess following expected Markovnikov attack at the central carbon by the metal salt. The yield of cyclopentenones was generally higher with acetoxymercuration (49-79%) compared to acetoxythallation (25-68%). Compounds of similar substitution pattern as were investigated with epoxidation (Scheme 31) were prepared. Mechanistically the reaction is remarkable for the facility of solvodemercuration (Scheme 33). Intermediates such as (69) or (70) lose elemental mercury spontaneously in the acetic acid medium to give the 2-cyclopentenones. 

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