Abstract
A short isoprenylation protocol starting from non‐conjugated N‐Boc‐N‐(1,1‐dimethylallyl)hydrazones was developed utilising Thomson's traceless bond construction. This type of [3,3]‐sigmatropic rearrangement is catalysed by the Brønsted acid triflimide and liberates only gaseous by‐products. The required N‐Boc‐N‐allylhydrazine precursor is available in three steps starting from a known diazene using biocatalytic aldol addition and Tebbe olefination as key steps. Allylhydrazones are prepared via condensation with appropriate aldehydes. Scope and limitations of the [3,3]‐sigmatropic rearrangements are analysed.
Highlights
The [3,3]-sigmatropic rearrangement is a common but impressive tool for the formation of new C–C-bonds in synthetic chemistry.[1]
Later our group extended the scope to the synthesis of 1,1-disubstituted olefins (D, Scheme 1b), bearing an isopropyl group in 1-position, which resulted in a methylene branched end, a motif which is found in the side chains of steroidal natural products, e.g. episterol.[4]
In the same year we reported the synthesis of terminal vinylsilanes (F, Scheme 1c) using traceless bond construction (TBC), which opened a new route to diversely substituted olefins.[5]
Summary
The [3,3]-sigmatropic rearrangement is a common but impressive tool for the formation of new C–C-bonds in synthetic chemistry.[1] In 1973 Stevens showed that N-allylhydrazones undergo such a rearrangement under release of N2 as well, but due to very harsh reaction conditions (300 °C) and low yields, this reaction was limited in its applicability.[2] For several decades, synthetic chemists did not see any real benefit of this unique rearrangement, until 2010, when Thomson and co-workers published the traceless bond construction (TBC), an improved variant of Stevens' [3,3]-sigmatropic rearrangement, working with N-Boc-N-allylhydrazones (A, Scheme 1a) and catalytic amounts of the Brønsted superacid triflimide (HNTf2).[3] It was possible to lower the temperature of the rearrangement to 125 °C and the yields of the products could be increased. In the same year we reported the synthesis of terminal vinylsilanes (F, Scheme 1c) using TBC, which opened a new route to diversely substituted olefins.[5]
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