Protecting groups facial selectivity

As shown in Fig. (7), the relative configurations for the diastereo-meric adducts were determined by NOESY correlations and coupling constants for ring protons. All the IMDA reactions proceeded with high endo/exo selectivity (A+B/C+D), while the n-facial selectivity of the endo-adducts (A/B) depended on the protecting group at C-l. The C-l alkyl- and acyl-protected substrates (88, 95, or 96) and non-protected substrate 89 provided the undesired cycloadduct B preferentially... 

The photochemical cycloaddition of ethene to the bis-butenolides (20) has been examined in an attempt to establish the influence of the ether-protecting groups of the diol system. Generally only two adducts are formed as can be seen from the results shown for the appropriate structures. The most effective ether protecting group is the trimethylsilyl function and here the facial selectivity yields predominantly the anti,anti adduct (21). With the unprotected systems (20, R = H), there is virtually no selectivity and in this case the three adducts (21), (22) and (23) are formed. Irradiation of the butenolides (20a) and (20b) in the absence of ethene leads to intramolecular hydrogen abstraction (a Norrish Type II process) with the formation of the products (24a) and (24b) in 79% and 76%, respectively. 

Unprotected a-hydroxy- or a-amino amides 3b and 3d also rearrange smoothly, probably facilitated by the intermediate formation of a dianionic species 572. auh-Products are either not detected or obtained in < 1 % yield. The facial selectivity ranges from 6.6 to 8.1. Protection of the hydroxy group, e.g., 3c, results in a decrease of both facial selectivity and chemical yield. 

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