Role of heteroatoms in diastereofacial control in cycloaddition to a dissymmetric cyclohexa-1,3-diene moiety in a polycyclic framework. Remarkable stereodirecting influence of distal protective groups

Diels-Alder cycloaddition to several derivatives of a facially dissymmetric diene, the hexacyclo-[7.5.1.0.(1,6)0.(6,13)0.(8,12)0(10,14)]pentadeca-2,4- diene-7,15-dione 4a, with a variety of dienophiles such as singlet oxygen, N-phenyltriazolinedione, dimethyl acetylenedicarboxylate, maleic anhydride, and N-methylmaleimide has been studied. The stereochemistry of the resulti ng adducts has been unambiguously secured by H-1 and C-13 NMR spectral data , chemical correlations, and X-ray crystal structure determination. While a variety of dienophiles undergo [4 + 2]-cycloadditions with 4a predominantl y from the carbonyl face, protection of the carbonyl groups in 4a as simple mono- or bis-acetals 4b-e or thioacetals 9a,b leads to complete reversal i n selectivity, favoring addition from the cyclobutane face, with heterodien ophiles and acetylenic dienophiles. The reversal in selectivity observed in mono- and bis-acetals 4b-e has been attributed to unfavorable electrostati c interaction between the oxygen atom and the incoming dienophile. Whereas, in the case of thioacetals 9a,b, apart from unfavorable electrostatic inte ractions, Cieplak-type hyperconjugative interactions have to be given due c onsideration in order to account for the observed selectivities. Our studie s highlight the role of simple protective groups (acetals in the present ca se) in modulating diastereoselection during [4 + 2]-cycloadditions.