Verkade’s bases

Typical structure of Verkade s base and its basicity due to trans-annular P-N... 

Given the limitations of the previous strategy, other research groups have introduced polycycHc propellers to prevent epimerization of the phosphorus center. Particular examples are Verkade s bases, also described as footballshaped proazaphosphatrane molecules (97JOC3677, 98JOC3691, 98JOC 10057). If these molecules do not exhibit any hehcal shape, they are considered as very strong bases due to the stabiHty of 102 when the free phosphine 101 reacts with a proton. Moreover no epimerization is observed with such structures (Scheme 27). 

Kovacevic, B., Baric, D. and Maksic, Z.B. (2004) Basicity of exceedingly strong non-ionic bases in acetonitrile - Verkade s superbase and some related phosphazenes. New Journal of Chemistry, 28, 284-288. 

One drawback of the use of weak bases as in situ promoters for the Schollkopf oxazole synthesis is the long time typieally required to drive such reactions to completion. For example, the condensation of methyl isocyanoacetate (14) and 3,4,5-trimethoxybenzoyl chloride (19) in the presence of DBU provides only an 8% yield of oxazole 21 after 2 h at ambient temperature. This limitation can be overcome by employing Verkade s prophosphatrane superbase (20), which, in the case of the reaction of 14 and 19, results in near-quantitative formation... 

Thiourea inclusion complexes. As noted in the second paragraph beginning on 1, 1165, a Delft group (Verkade et al.s) reported an efficient separation of cis-and trarcs-4-isopropylcyclohexane-l-carboxylic acid based on their observation that the trans-acid forms a thiourea inclusion complex whereas the cis isomer does not. However, they failed to account for the difference. A simple explanation became... 

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