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Abstract
We have prepared novel [η4] and [η5]+ tricarbonyliron complexes from an unusual enantiopure cyclohexadiene ligand that possesses a quaternary stereocentre; this in turn is prepared through biotransformation of an aromatic ring. The cyclohexadiene ligand initially possessed two hydroxyl groups, both of which could be substituted with other functionality by means of an overall [η4] → [η5]+ → [η4] → [η5]+ → [η4] sequence. From six novel tricarbonyliron complexes which have been prepared, three have been characterised by x-ray crystallography. The reaction sequence we describe is potentially of relevance to the synthesis of analogues of the anti-influenza drug oseltamivir. In addition, the failure of an attempted addition of a bulky nitrogen nucleophile to an [η5]+ complex sheds light on the limits of reactivity for such additions. Thus, two bulky nucleophiles which are each known to add successfully to unencumbered [η5]+ complexes seemingly cannot be added sequentially to adjacent positions on the cyclohexadiene ligand.
Highlights
Dihydroxylation of an aromatic ring using a microorganism is a useful synthetic method, insofar as such a transformation is very difficult to achieve by conventional methodology [1]
[h4] / [h5]þ / [h4] transformations from complex 11 for the purposes of diversifying the cyclohexadiene ligand [14c]. Such reaction sequences have been reported previously for tricarbonyliron complexes derived from arene cis-diols of type 2 [12aed,f,g]
All of the products 19 and (±)-20 smoothly underwent oxidative decomplexation of the tricarbonyliron fragment, so giving a range of novel cyclohexadienes for use in synthesis. Both 19 and (±)-20 possess a residual acetoxy group, which could be induced to leave by treatment with Brønsted acid; this would lead to formation of another [h5]þ complex, which in turn could be treated with another nucleophile to give a further [h4]
Summary
Dihydroxylation of an aromatic ring using a microorganism is a useful synthetic method, insofar as such a transformation is very difficult to achieve by conventional methodology [1]. We subsequently sought deliberately to exploit [h4] / [h5]þ / [h4] transformations from complex 11 for the purposes of diversifying the cyclohexadiene ligand [14c] Such reaction sequences have been reported previously for tricarbonyliron complexes derived from arene cis-diols of type 2 [12aed,f,g]. All of the products 19 and (±)-20 smoothly underwent oxidative decomplexation of the tricarbonyliron fragment (except 20, Nu 1⁄4 H), so giving a range of novel cyclohexadienes for use in synthesis Both 19 and (±)-20 possess a residual acetoxy group, which could be induced to leave by treatment with Brønsted acid; this would lead to formation of another [h5]þ complex, which in turn could be treated with another nucleophile to give a further [h4]. On the basis of the above rationale, we sought to synthesise an analogue of oseltamivir with a substituent at C6; this paper describes our results in this regard
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