Group 15 systems rings

In the second group of ring-chain tautomeric interconversions, an open-chain system is transformed into a cyclic system through the intramolecular reversible addition of a functional group to a polar multiple bond lA IB 2A 2B 3A 3B and 4A 4B. The book (I) and this article deal with... 

The functional group in ring A is a ketone a planar system that takes in carbons adjacent to a ketone is the enol tautomer. We are using acid catalysis here, and that is appropriate for enolization. If we first protonate the carbonyl, then try moving electrons, we shall soon find that the proton at position 6 (standard steroid numbering) can be lost in generating a conjugated enol tautomer. In a reversal of this process back to the ketone, we can pick up a proton at position 6 from either face. In this case, protonation on the upper face allows the methyl substituent to take up the more favourable equatorial position. 

Specific examples of the application alkane elimination for the synthesis of either saturated or unsaturated group 13-group 15 ring systems are shown in Scheme 2.6 (for a more detailed discussion, see Sections 9.4.1 and 9.4.2). 

Within each of the main groups of ring systems, the reactivity chapters are arranged in the same way, as is described in Part 5 (Appendix A , Section 4). 

The aromatic intermediates are combined, with the incorporation of sulfur, to form higher molecular weight chromophore systems. If the intermediate contains both CH3 and NH2 groups, thiazole rings are presumably the linking group as well as the chromophore. 

In certain systems — especially in those carrying more than one donor or acceptor group — a ring expansion to five-membered heterocycles 11 occurs at relatively low temperatures. This process can be classified as a 1,3-sigmatropic shift and is most conveniently explained by the intermediacy of the 1,3-zwitterion 10. 

---