NEW TYPE OF CYCLIZATION OF ALPHA,BETA,CHI,PSI-UNSATURATED DIOIC ACID-ESTERS THROUGH TANDEM CONJUGATE ADDITIONS BY USING LITHIUM N-BENZYL-N-(TRIMETHYLSILYL)AMIDE AS A NITROGEN NUCLEOPHILE

Treatment of dimethyl (2E,6E)-2,6-octadienedioate with lithium N-benzyl-N-(trimethylsilyl)amide (LSA) gave 5-exo-trig ring closure products, methyl 3-(N-benzylamino)-2-(methoxycarbonyl)cyclopentane-1-acetates, through tandem conjugate additions. The related 6-exo-trig cyclization proceeded stereoselectively to give ethyl c-3-(N-benzylamino)-t-2-(ethoxycarbonyl)cyclohexane-1-acetate. In contrast with the 5- and 6-exo-trig cyclization, no 7-exo-trig cyclization occurred. The 5-exo-trig cyclization products were converted into 2-(methoxycarbonyl)-2-cyclopentene-1-carboxylic acid methyl ester in an excellent yield. 5-exo-trig cyclization of the unsymmetrical dienedioate consisting of crotonate and (E)-2-methyl-2-butenoate units proceeded regioselectively through conjugate addition of LSA to the crotonate part. Similar regioselectively was observed in the case of 5-exo-trig cyclization of the dienedioate possessing crotonate and (E)-3-methyl-2-pentenoate units. Total syntheses of the physiologically active cyclopentane monoterpenes (+/-)-dihydronepetalactone and (+/-)-isohydronepetalactone has been accomplished by this cyclization strategy. In addition, it has been demonstrated that LSA is a more efficient nitrogen nucleophile than LDA to cyclize omega-halo-alpha,beta-unsaturated esters.