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

Chapter 2 – Dearomatising additions to aryl oxazolines
Chapter 2 – Dearomatising Additions to Aryl Oxazolines
2.1 Establishing a New Reaction
Ph
Ph
Ph
Ph
Ph
O
N
Ph
SiO 2
O
N
i) RLi (1.5 eq)
THF:DMPU 25:1
−78 °C
ii) MeI
O
N
Me
R
100a
prepared to attempt
dearomatising cyclisation
101a
102a
R = i-Pr, 102a, 61%
s-Bu, 102a', 91%
(dr 4:1)
Scheme 2.1 – first dearomatising additions by Purewal 63
Whilst attempting the dearomatising cyclisation of benzoyl aziridine 100a, Purewal
unintentionally prepared oxazoline 101a. When subjected to the conditions of
dearomatising cyclisation 62 this material underwent nucleophilic attack by the
organolithium to give dienes 102a’. This was successfully repeated with i-PrLi to give
a single diastereomer, confirming that this reaction is highly diastereoselective,
however dienes 102a were described as aziridines in his thesis. 63 A crystal structure
later obtained by Cabedo confirmed the relative stereochemistry of the diene, and
showed it to be an oxazoline rather than an aziridine, 64 making a clear link to the
chemistry Meyers had developed with 2-naphthyl- and pyridyloxazolines (section
1.3.2).
Personal communication with Meyers confirmed that during the oxazoline research
programme in his laboratories he had been unable to achieve dearomatising addition to
benzenoid aromatics. 36 With this news it was clear that this might be a novel reaction,
differing from Meyers’ in both the oxazoline and co-solvent. This was confirmed by
treating Meyers’ anisoyloxazoline 104b under both sets of conditions. 31
51