A Route to Carbasugar Analogues - Jonathan Clayden - The ...

2.4 – Mechanistic discussion
The tethered oxazoline 101j was treated under the reaction conditions, giving a forest
green solution. Mass recovery from the reaction was poor, but the isolated products
were very interesting.
Ox*
i) i-PrLi (2 eq)
THF, DMPU
−78 °C
Ox*
Ox*
Ox*
101j
O
ii) MeI
O
102j
O
O
10% 5% 20%
102j*
142 dr 2:3
Scheme 2.31 – dearomatising addition of allyl tether
The major products of the reaction are believed to be two diastereomers of bicycle 142,
with a combined yield of 20%. However, two discrepancies exist in this assignment;
whilst the 1 H spectra NMR of the separated diastereomers appears to be show single
compounds, adjacent half-intensity peaks appear in the 13 C NMR, resembling a 1:1
mixture of diastereomers. Furthermore the 1 H NMR spectra contain many high
multiplicity peaks with small couplings (1-2 Hz). These spectra are provided in
appendix A.
Dearomatised adduct 102j and its isomer 102j* were characterised by having almost
identical 13 C and 1 H spectra to adduct 102c, which had been identified by crystal
structure, only differing in the pendent allyl system. Importantly, no other products
could be identified in the crude NMR. Unfortunately only 15% of the dearomatised
adducts 102j were recovered, compared with 54% of 102c. Whilst diminished yields
were anticipated due to the reactivity of the allyl group, it means that we cannot be
sure that some tetrahydrobenzofuran 136 was not formed, and cannot exclude two
reaction pathways. As indicated in Scheme 2.28, the absence of a trapped radical does
not exclude electron transfer, but simply gives an upper limit on the lifetime of a
radical anion ‡ETj of around 10 ns. This allows us to rule out a slow RC step which
might have involved diffusion of i-Pr• out of the solvent cage.
Whilst much controversy has surrounded the use of allyl tethers as mechanistic probes,
in Newcomb’s critique on the practice he concludes that the only mechanistic certainty
they provide is in the absence of radical trapping. 92 Although the poor yield means we
cannot be certain, we have shown that free radical intermediates are unlikely.
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