Spiro dioxolanes, formation

Evidence for the formation of Meisenheimer-type adducts from a purine system has been obtained by Liotta in two cases.43 The addition of t-BuO to 6-(2-hydroxyethoxy)-9-methoxymethyIpurine in t-BuOH, as monitored by H-NMR spectroscopy, causes an upfield shift of both pyrimidine and imidazole ring protons and the conversion of two absorptions of the methylene protons of the CH2CH2OH group to a broad absorption of the dioxolane ring, in agreement with the formation of the spiro adduct 19. Similarly, adduct 20 was formed from 6-methoxy-9-methoxymethylpurine by slow reaction with MeCT in 7-BuOH. 

The mechanism of the aldehyde homologation is predicted to occur through a multistep process that consists of the concerted formation of an A-thiazolium 2-yUde intermediate (10), followed by nucleophilic addition of this intermediate to another aldehyde molecule (11). Loss of the first aldehyde gives the addition product (eq 20). The observation of the spiro(thiazoline-2,4 -dioxolane) (12) by NMR in a kinetic study provides additional evidence for this mechanism. The conversion of compound 12 into the final product should proceed through the loss of two molecules of aldehyde and the transfer of the silyl group to the oxygen atom in the C-2-alkyl moiety. 

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