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 resulting adducts has been unambiguously secured by (1)H and (13)C NMR spectral data, chemical correlations, and X-ray crystal structure determination. While a variety of dienophiles undergo [4 + 2]-cycloadditions with 4a predominantly 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 in selectivity, favoring addition from the cyclobutane face, with heterodienophiles and acetylenic dienophiles. The reversal in selectivity observed in mono- and bis-acetals 4b-e has been attributed to unfavorable electrostatic interaction between the oxygen atom and the incoming dienophile. Whereas, in the case of thioacetals 9a,b, apart from unfavorable electrostatic interactions, Cieplak-type hyperconjugative interactions have to be given due consideration in order to account for the observed selectivities. Our studies highlight the role of simple protective groups (acetals in the present case) in modulating diastereoselection during [4 + 2]-cycloadditions.