A Staudinger-based synthesis of a stable acylal precursor to 2-formyl-3-methylene-1-pyrroline has been developed. Subsequent acetyl transfer to phenoxide in dry THF generates a mono acetyl potassium hemiacetal that decomposes to the target aldehyde in situ. While the product cannot be concentrated without decomposition, it can be characterized in solution and engaged in diverse reactions. These include nucleophilic substitution, cycloadditions, and an unprecedented Lewis acid-base-mediated dimerization forming pyrrolo-pyrroline scaffolds.
Lotuzas et al. (Tue,) studied this question.