Departure of Pyridines

The data which had been held to support ALPH are now thought to arise from either least motion effects, which are minimised in reactions with early or late transition states,or systematic bias in the choice of system. In the decomposition of tetrahedral intermediates, an upper limit of 8kJmol has been placed on any antiperiplanar lone pair effect in any system. Although discredited, the ideas are superficially appealing and even at the time of writing (2007) are occasionally invoked by the non-specialist. 

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In contrast to the preparations of 1,2-dithiolane derivatives described in this section, the sulfurization of /3-dithio-lactone 303 (Scheme 56) with 2 equiv of elemental sulfur provides a single example of formation of a 1,2-dithiolane-3-thione derivative 305 <2006H(68)2243>. An ionic intermediate 304 has been invoked to explain a ring enlargement via simultaneous departure of pyridine and migration of sulfur. 

Figure 3.8 Elfects of sugar ring substitution on departure of pyridine.

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