Abstract Reduction of the acetoxymethylene ketone 1b yielded a mixture of two epimeric diols 3a and 4a , which were converted into the p ‐toluenesulfonates 3d and 4d . Solvolysis of 3d in dry acetic acid afforded 5b after inversion at C‐17. As an explanation for the formation of 5b , it was assumed that the cyclic cation 8 may be an intermediate. This was confirmed by methanolysis of 3d leading to the orthoester 10 . The neighbouring group participation (AcO‐6) observed during solvolysis of 3d was completely absent in the case of 4d , solvolysis of which yielded 17β‐acetoxy‐16α‐acetoxymethyl‐3‐methoxyestra‐1,3,5(10)‐triene ( 4b ) with retention in both acetic acid and methanol.

Neighbouring group participation In organic chemistry, neighbouring group participation (NGP, also known as anchimeric assistance) has been defined by the International Union of Pure and Applied Chemistry(IUPAC) as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma or pi bond contained within the parent molecule but not conjugated with the reaction centre. When NGP is in operation it is normal for the reaction rate to be increased. It is also possible for the stereochemistry of the reaction to be abnormal (or unexpected) when compared with a normal reaction. While it is possible for neighbouring groups to influence many reactions in organic chemistry (e.g. the reaction of a diene such as 1,3-cyclohexadiene with maleic anhydride normally gives the endo isomer because of a secondary effect {overlap of the carbonyl group π orbitals with the transition state in the Diels-Alder reaction}) this page is limited to neighbouring group effects seen with carbocations and SN2 reactions. ## NGP by heteroatom lone pairs In this type of substitution reaction, one group of the substrate participates initially in the reaction and the