From non-heterocyclic precursors

Photolysis of phenacylamines (32) gives azetidin-3-ols (33) as minor products (66CC289). Higher and more consistent yields are obtained from photolyses of a-N-alkylamidoaceto-phenones (34), which gives N-tosylazetidines (35) in 74-95% yield (71JA2793). The key step 

Dialkylation of an amine or sulfonamide with a 1,3-dihalide provides a further route to azetidines 79CRV33l, 64HC( 19-2)88 5). Examples of this approach are the formation of N-tosylazetidine from tosylamide and l-bromo-3-chloropropane and the formation of N-alkylazetidinyl esters (36). The latter reaction works well except for R=Me the former provides a useful route to azetidine since the tosyl group can be removed by reductive methods. 

There are few examples of formation of simple azetidines from [2 + 2] fragments, although this type of approach is important for certain azetidine derivatives, e.g. azetidin-2-ones (see Section 5.09.3). Nitrogen analogues of the Paterno-Buchi route to oxetanes are rare an example involves the photoaddition of 3-ethoxyisoindolone (37) to the enol ether (38) (75JA7288, 72CC1144). 

---

The final important category of [4 + 2] cyclization is the Diels-Alder and other [4 + 2] cycloaddition reactions. This topic has been the subject of intense study for many years and continues to attract the attention of many researchers, as in a great many instances the reactions involved constitute excellent procedures for the synthesis of both heterocyclic and non-heterocyclic compounds. The discussion in this section is restricted to processes which result in the formation of heterocyclic compounds from non-heterocyclic precursors, although of course a heterocycle may be present as a substituent in either the diene or ene component but have no significant or controlling effect in the cycloaddition reaction. 

The more generally important approaches to quinoline and isoquinoline compounds from non-heterocyclic precursors are summarised in this section. 

Indoles are usually prepared from non-heterocyclic precursors by cyclisation reactions on suitably substituted benzenes they can also be prepared from pyrroles by construction of the homocyclic aromatic ring, and from indolines by dehydrogenation. 

---