An unexpected conversion of 2E-furfurylidene-3-oxo-24-nor-allobetulin to 23-nor-allobetulins

[Display omitted] •3-Oxo-24-nor-allobetulin was modified using Claisen-Schmidt condensation.•2E-Furfurylidene-3-oxo-24-nor-allobetulin and two byproducts were isolated.•They are 3-oxo-4α-hydroxy- and 3β,4α-dihydroxy-2E-furfurylidene-23-nor-allobetulins.•The possible mechanism of inversion at C4 and reduction of C3-oxo-group are explained. A series of 2E-furfurylidene-23-nor- and 24-nor-allobetulins has been synthesized by the Claisen-Schmidt condensation and conditions of their formation were studied in detail. It was found that among an expected 2E-furfurylidene-3-oxo-24-nor-allobetulin 4 two byproducts holding 3-oxo-4α-hydroxy- 5 and 3β,4α-dihydroxy- 6 substituents were formed, which could become the main products under the change of reaction time and amount of the base solution. Moreover, a conversion of individual 2E-furfurylidene-23-nor-3-oxo-4α-hydroxy- 5 into 2E-furfurylidene-23-nor-3β,4α-dihydroxy-derivative 6 under the treatment with the base solution was observed. An inversion of the configuration at C4 from 24-nor- to 23-nor-allobetulins for compounds 5 – 7 was proved by the NMR spectra. The probable explanation of compound 5 formation includes oxidation by atmospheric oxygen to 4-hydroperoxide, which was further transformed into 4-hydroxy-group. In the presence of the base the reduction C3(=O)-function of compound 5 occurs like Meerwein– Ponndorf–Verley reaction to give compound 6. As a result, a difference in the reactivity of native allobetulin scaffold and 24-nor-allobetulin in the Claisen-Schmidt condensation was observed and a first case of conversion 24-nor- to 23-nor-allobetulin derivatives was described.