This communication by Furstner and co-worker reported a novel rearrangement of methylenecyclopropanes 3 for the construction of variously substituted cyclobutenes and derivatives.
I am particularly interested in the preparation of cyclopropylidene bearing substrates 3. In this paper, the cyclopropylidene bearing substrates 3 were generated by a new variant of the Julia-Kocienski olefination starting from aldehyde 1 and cyclopropyl sulfone 2 (Scheme 1, For procedure, see Supporting Information). Initially, I reproduced the preparation of Julia-Kocienski olefination reagent 2 (a 4-step process) and the corresponding Julia-Kocienski olefination to 5. The procedures are very reliable and good yielding for this case. However, when this methodology was applied to ketone substrates instead of aldehydes (in this paper), the efficiency of the reaction was dropped significantly in terms of yield (typically 30-40%). Then I modified their procedure for Julia-Kocienski olefination (Cs2CO3, 70 Celsius degrees, THF/DMF, 40 h) to a low-temperature variant (KHMDS, –78 Celsius degrees, THF, overnight) and got much improved yield (typically 70-80%) for ketone substrate.
The modified procedure is decribed as following:
To a solution of ketone or aldehyde (1.0 mmol) and the Julia-Kocienski reagent 2 (276 mg, 1.2 mmol) in THF (10 mL) at –78 Celsius degrees was added potassium bis(trimethylsilyl)amide (1.0 M in THF, 1.3 mL, 1.3 mmol). The reaction was allowed to warm to room temperature and stirred at room temperature overnight. A saturated aqueous NH4Cl solution (20 mL) was added to quench the reaction and the reaction mixture was extracted with Et2O (3 × 30 mL). The combined organic layers were washed with brine (30 mL), dried over Na2SO4, filtered and evaporated under reduced pressure. The residue was purified by silica gel column chromatography to give the title cyclopropylidene bearing substrates in pure form.
It is also worth mentioning that this modified procedure worked well for aryl aldehyde and nearly all ketone substrates. However, it exhibited low efficiency for aliphatic aldehyde substrates. For aliphatic aldehyde substrates, I would suggest using Wittig reaction as an alternative (the corresponding Wittig reagent is commercially available).