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C7H603, M r= 138.1, monoclinic, P21/c, a=

6.36 (1), b= 9.58 (2), c= 9.71 (1) A,#= 75.13 (2) from diffractometer measurements (Mo K5 radiation), V= 572A 3, Z= 4, D m= 1.60, D c= 1.60 Mgm-3, F (000)= 288,#= 0.08 mm-~, crystal dimensions 0.5 x 0.2 x 0.1 mm. R= 0.041 for 1124 observed reflexions. This highly strained tricyclic molecule has an unusually short transannular O... C= O interaction [2.651 (2) A] and a significant [0.052 (1) A] displacement of C (3) from the plane of the carbonyl group.

Introduction. We are undertaking a survey of the geometries of intermediates in a recent synthesis of prostaglandins(Newton, Howard, Reynolds, Wadsworth, Crossland & Roberts, 1978) to determine the factors involved in observed stereo-and regioselectivity (Murray-Rust, Murray-Rust & Newton, 1979). We have already noted the possibility of a transannular O... C= O interaction (Brown, Glen,