Scheme 6 Tandem 4n-6n electrocyclic concerted process at low temperature [70] and under photochemical conditions [71]

If the substituent is allylic and hence homoconjugative, intramolecular [2 + 2] cycloaddition is progressive to form bicyclo[3.2.0]heptenone as shown in Fig. 4 [19-23]. This is the second pattern of the ring transformation based on squaric acid.


R4 OR6r5

R2 R1

Fig. 4 Intramolecular [2 + 2] cycloaddition to bicyclo[3.2.0]heptenone [21]

The third pattern was developed by Paquette, who studied the possibility of the concerned reaction extensively and established the cascade rearrangements route [72]. The scenario of the cascade rearrangement is:

1. 1,2-Addition of a pair of alkenyl anions (either the same or different) to a squarate ester in an anti and/or syn manner

2. Charge-driven 4n conrotatory opening to coiled 1,3,5,7-octatetraene followed by 8n recyclization for antz-adduct, and straightforward oxy-Cope rearrangement for syn-adduct

3. Transannular Aldol condensation for the final ring closure to bicycles

The prototype as shown in Fig. 5 was extended to more sophisticated molecular design such as a polyquinane skeleton.

2 X CHz=CHLi

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