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Four members of a novel class of natural 'phlobaphene' condensed tannins, representing the products of stereospecific ring-isomerization of those 2,3-trans-3,4-trans-( -)-fisetinidol units present in 'conventional' [4,8]-biand [4,6 : 4,8]-tri-flavanoid profisetinidins, are defined as functionalized 8'9-trans-9'1 O-cis-3,4,9,1 O-tetrahydro-2 H,8H-pyra no [ 2,3-h] c h ro m en e ( 1 ) a n d the h o m o I og o u s h exa h y d rod i p y r a no [ 2'3-ff 2 ' ,3 'h] c h ro m e n e (5) by n u cl e a r Overhauser effect difference spectrometry.

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Condensation of (+) -(2R) -2,3-trans-3,4-trans-flavan-3,3',4,4',7-pentaol [ (+) -mollisacacidin] $ with excess of (-) -(2R) -2,3-trans-flavan-3,3',4',7-tetraol [ (-) -fisetinidol] proceeds beyond the expected biflavanoid range to generate significant yields of both the ' linear ' [4,6 : 4,6] -2,3-trans-3,4-cis : 2',3'trans-3',4'-trans : 2",3"-trans-trifisetinidoI and the first ' branched ' [4,6 : 4,8 : 4,6] -2,3-trans-3,4-cis :2',3'trans-3',4'-trans : 2",3"-trans-3",4"-trans : 2"',3"'-tran~-tetrafisetinidol.The products indicate a selective condensation sequence due to differing steric constraints, operative at competing nucleophilic centres in each intermediate substrate, assisted by hyperconjugative effects.

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