“…However, in terms of the steric effect of /3-substituents compared with that of 7-substituents [cf. the rule of six (240) ] a fairly complete rationalization is apparent (49). Thus the propionamides b, d, and e have no ßmethyl groups and on steric grounds would be most labile to Sn2 reaction giving the three mono-, three di-, and one tricarboxylic acids observed.…”