Heterocyclic rings unsaturated five-membered

Mathey F (2002) In Maas G, Regitz M (eds) Science of synthesis hetarenes and related ring systems fully unsaturated small-ring heterocycles and monocyclic five-membered eetarenes with one heteroatom, vol 9. Thieme, Stuttgart, p 553... 

Fully unsaturated five-membered ring systems with one other heteroatom adjacent to phosphorus have only been known only since 1986 <86CB4io> and therefore no representative is contained in Comprehensive Heterocyclic Chemistry, First Edition (CHEC-I) <84CHEC-i(i)523>. They include now... 

Zhang [343] applied the previously described pK estimation protocol of Zhang, Baker, andPulay [172] to several sets of heterocyclic bases with impressive results. For a large set of 115 unsaturated five-membered ring bases Zhang found... 

The intermediate in the synthesis of aminoethylethanolamine-derived amphoterics is a heterocyclic imidazoline with an unsaturated five-membered ring system containing an amine and an imine group (Figure 15.13). 

Pyrrole (two r s, one /) and imidazole are /ive-membered heterocycles, yet both have six tt electrons and are aromatic. In pyrrole, each of the four. sp2-hybridized carbons contributes one tt electron, and the sp2-hybridized nitrogen atom contributes the two from its lone pair, which occupies a p orbital (Figure 15.9). Imidazole, also shown in Figure 15.9, is an analog of pyrrole that has two nitrogen atoms in a five-membered, unsaturated ring. Both nitrogens are sp2-hybridized, but one is in a double bond and contributes only one electron to the aromatic tt system, while the other is not in a double bond and contributes two from its lone pair. 

Besides the bond-pair cheletropic disconnection of oxiranes and aziridines to an alkene and "atomic oxygen" (from a carboxylic peracid) or a nitrene, respectively, and the hetero-Diels-Alder cycloreversion, of special interest are the 1,3-dipolar cycloeliminations of five-membered rings [-(34-2)] leading to 1,3-dipoles and an unsaturated acceptor or dipolarophile. So large is the number of different five-membered heterocyclic systems resulting from 1,3-dipolar... 

Heterocyclic carboxylic esters (708) are as reactive as formate and acetate esters. Extended to 1-substituted biguanides, the reaction is promoted by metal alkoxides. Since the heterocyclic ring must be five-or six-membered and contains a conjugated system of double bonds, the reaction resembles formally that involving a,p-unsaturated esters (see below). Several successful examples are on record (599, 606). 

Lactones are cyclic esters derived from lactic acid (CsHeOs). They are constituents of many essential oils and plant volatiles. They contain a heterocyclic oxygen next to a carbonyl function in a five or more membered ring that is saturated or unsaturated. Those with a five-membered ring are called y-lactones, e.g. y-angel-ica lactone 148, whereas compounds containing a six-membered ring are called (5-lactones, e.g. (5-valerolactone 149 (Structure 4.45) [1-4, 6, 9, 22, 23,29, 62]. 

Among the heterocyclic compounds, there are aromatic, e.g. pyridine, as well as nonaromatic, e.g. tetrahydrofuran, compounds. Similarly, there are saturated (e.g. tetrahydrofuran) and unsaturated (e.g. pyridine) heterocyclic compounds. Heterocycles also differ in their ring sizes, e.g. pyridine has a six-membered ring, whereas tetrahydrofuran is a five-membered oxygen-containing heterocyclic compound. 

Thiophene is a sulphur-containing five-membered unsaturated heterocycle. The lone pair electrons of the sulphur are in the 35 orbital, and are less able to interact with the tt electrons of the double bonds. Therefore, thiophene is considered weakly aromatic. Acetylenic thiophene is found in some higher plant species. However, the thiophene ring is present in many important pharmaceutical products. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

Simple ring-opening reactions ([5 -> 5] isomerizations), which might be considered as the reversal of well-known cyclization reactions, are rather uncommon in five-membered ring compounds, and only few examples are known for unsaturated heterocycles such as pyrrole (8), dihydrofurans (9, 10), and isoxazoles (11). 

American practice is to use the terminations -ane for saturated and ene for unsaturated compounds in both the hydrocarbon and the heterocyclic series. British usage is essentially the same for hydrocarbons, but for heterocyclic molecules the final e is omitted in fact, a terminal e is used in general only for hydrocarbons, bases, and the five-membered ring ending ole. This leads in particular to some differences in the spelling of both trivial and systematic names for one-ring heterocycles e.g., thiophen, oxiren, dioxaphospholan (British) thiophene, oxirene, dioxaphospholane (American). 

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