Equatorial side-chain

We envisioned that the B ring of cylindrospermopsin (1) could be prepared by an intramolecular SN2 reaction of the guanidine of 4 on the bro-moketone. Equilibration would provide the desired equatorial side chain. Reduction of the ketone, deprotection and sulfation would complete the synthesis of cylindrospermopsin. Addition of acetylene 5 to pyrimidine... 

The ring of ABA can theoretically have at least two minimum-energy conformers one is an envelope (or half-chair) with the axial side-chain, 1-i, and the other is an inverted envelope (or half-chair) with the equatorial side-chain, 1-ii [Fig. (3)] [42]. The crystal structure that has been reported for ABA [43,44] reveals that its ring is 1-i, and the nuclear Overhauser effect (NOE) experiments have suggested that the ring of... 

Model 21m also had three different types of ring at the minimum-energy points two envelopes with axial and equatorial side-chains (21m-i and 21m-ii) and one boat with the equatorial side-chain (21m-iv). The axial/equatorial ratio was 59 41, and the free energy difference was almost zero. This may be due to the cyclopropyl ring that is syn to the side-chain. The axial side-chain would be repulsive to the cyclopropyl ring. 

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