Compounds with Four Heteroatoms

For compounds with three or four heteroatoms in the ring the number of measurements is much fewer, and these are summarized in Table 1. 

Table 1 X-Ray Structures of Compounds with Five-membered Rings and Two, Three or Four Heteroatoms ...

Only a few representatives of scvcn-membcred ring compounds with maximum unsaturation containing four heteroatoms that fall within the scope of this section have been reported. 

The present review is concerned with four-membered heterocycles that contain one sulfur atom either alone or associated with another heteroatom. The classes of compounds that will be discussed are the thietanes (1) and thietes (2), the 1,2-oxathietanes (3), the / -sultines and -sultones (4 and 5), the 1,2- and 1,3-thiazetidines (6 and 7) and thiazetes (8), the 1,2- and 1,3-thia-phosphetanes (9 and 10), and the 1,3-silathietanes and 1,3-selenathietanes (11 and 12). 

Four-membered rings with two heteroatoms are a unique and important class of compounds if for no other reason than for their medicinal, agricultural and industrial use. 

Seven-membered rings with four and more heteroatoms attract attention because of their biological activity. An important class of compounds is that of the seven-membered cyclic polysulfides with five sulfur atoms, which are well known as lenthionine and varacin. 

Optical activity was first observed with organic compounds having one or more chiral carbon atoms (or centres) (i.e. a carbon substituted with four different groups). In the structures (1) to (17) the chiral carbons are specified with an asterisk. Subsequently compounds having chiral centres at suitably substituted heteroatoms (e.g. silicon, germanium, nitrogen, phosphorus, arsenic, sulphur, etc.) were also synthesised. Molecular dissymmetry, and hence chirality, also... 

The compounds with five-membered rings containing three or four heteroatoms including at least one phosphorus were first systematically reviewed in Chapter 4.22 of CHEC-II(1996) by Schmidpeter <1996CHEC-II(4)771>. In CHEC(1984), only few examples of 2//4,2,3-diazaphospholcs were mentioned in the general chapter on phosphorus heterocycles <1984CHEC(1)493>. 

This section provides an update on the synthesis, properties, and reactivity of heterophospholes with three or four heteroatoms. The section structure and compound classification are the same as in the original Chapter 4.22 of CHEC-II(1996) <1996CHEC-II(4)771>. 

We might first think of removing the structural heteroatom—the ring nitrogen. With reductive amination in mind we might consider imines from 52 or amides from 53. But these compounds have four different carbonyl groups and obviously problems of selectivity arise. 

Since 1,3-diselenetane was first synthesized in 1920 <1920JCS1456>, various four-membered rings with two heteroatoms including selenium or tellurium have been prepared. 1,2- and 1,3-Diheteroatom-four-membered cyclic compounds, as shown below, are summarized up until CHEC-II(1996) <1996CHEC-II(1B)1139>. 

All four-membered heterocycles with three heteroatoms can be classified into four types compounds with three similar heteroatoms 1 symmetrical 2, and unsymmetrical 3 heterocycles with two different heteroatoms and derivatives with three different heteroatoms 4 <1996CHEC-II(1B)1189>. 

The small class of compounds that have a carbon atom with four bonds to heteroatoms is related to CO2 and best described as at the carbon dioxide oxidation level. 

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