Si/Ge bioisosterism

Replacement of the central carbon atom by a silicon atom in the (R)-enantiomers of the hydroxymethyl analogue of HHD (3->4) and THP (8- 9) lowered affinity and stereoselectivity, whereas the receptor selectivity for compounds 3 and 4 was unchanged. In contrast, the corresponding (R)- and (S)-enantiomers of the Si/Ge analogues exhibited very similar affinities to the different muscarinic receptors indicating a strongly pronounced Si/Ge bioisosterism. 

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