Antiperiplanar lone pair hypothesis ALPH

It has also been called the antiperiplanar lone-pair hypothesis (ALPH). For a reinterpretation of this factor in terms of the principle of least nuclear motion (see 15-10), see Hosie,L. Marshall,P.J. Sinnott,M.L.y. Chem. Soc.,Perkin Trans. 2,1984,1121 Sinnott, M.L. Adv. Phys. Org. Chem., 1988, 24, 113. 

Failures of the antiperiplanar lone pair hypothesis (ALPH). 120... 

Figure 3.10 Ideas behind the theory of stereoelectronic control or antiperiplanar lone pair hypothesis (ALPH) Antiperiplanar (sp ) lone pairs are shown shaded.

The hypothesis gained considerable plausibility by appearing to be a simple extension of what was already accepted. Thus, an antiperiplanar lone pair of electrons can be considered to play essentially the same role as the pair of electrons in the breaking C—H bond in an E2 elimination, and the strong preference for anti-stereochemistry in such reactions had long been known. Moreover, the frontier-orbital explanation of the anomeric effect was receiving support at the expense of the antecedent electrostatic explanation, and the ALPH merely applied the same ideas to transition states. 

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