Sulfur systems, acyclic

An acyclic NCCN system in which the N-C links may be sp, sp2 or sp3 hybridized reacts with sulfur monochloride or sulfur dichloride to form the appropriate 1,2,5-thiadiazole (68AHC(9)107, 67JOC2823). Thus, diiminosuccinonitrile (384) gives 3,4-dicyano-l,2,5-thiadiazole (385) (72JOC4136). Tables 16 and 17 in CHEC 4.26 list the various substituted systems prepared in this fashion. 

The oxygen-sulfur interchange reaction was extended to an acyclic anhydride system. High conversion of acetic anhydride to a mixture of thioacetic acid and acetic acid was obtained employing alkali metal... 

A direct attack of nucleophiles on the sulfur atom of the sulfone or sulfoxide group in acyclic or large-ring sulfones and sulfoxides is rather rare, or unknown, excluding metal hydride reductions and/or reductive deoxygenations. The situation is completely different in the three-membered ring systems. 

Several trends have emerged in the extensive carbon-13 NMR spectroscopy data that have been accumulated for sulfones and sulfoxides. Based on many studies of cyclic systems—particularly five- and six-membered ring sulfur compounds—these trends were shown to generally apply equally to both the cyclic and acyclic systems . Thus (a) oxidation of a sulfide to a sulfone results in a 20-25 ppm downfield chemical shift for sp -hybridized a-carbon atoms and 4-9 ppm upfield shift for / -carbons , and (b) there is very little difference between the chemical shifts of a-carbon atoms of sulfones and sulfoxides despite the difference in the inductive effects of these two functional groups . A difference is observed, however, in the H chemical shift of related cyclic sulfoxides and sulfones . 

Thiirene dioxides readily react with an entire spectrum of enamines to provide novel acyclic and cyclic systems . These products result mostly from carbon-carbon or carbon-sulfur bond cleavage in the intermediate fused thiirane dioxide 167 (equation 67). 

The participation of sulfoxy groups can be used to control the stereochemistry in acyclic systems. In the reaction shown below, the internal sulfoxide captures the bromonium ion and then undergoes inversion at sulfur in the hydrolytic step. 

Simultaneous formation of a two-ring system from acyclic intermediates has also been reported. For example, the reaction of carbonyl-stabilized sulfur ylides with nitrile imines afforded furo[3,2-c]pyrazoles (69TL853). 

The most commonly employed transformations for the construction of five-membered rings containing three sulfur or oxygen atoms in the 1,2,4-positions are shown in Table 11. These have attracted more interest than the syntheses from acyclic components. The rearrangement of a 1,2,3-trioxolane (primary ozonide) to a 1,2,4-trioxolane (secondary ozonide) is the most generally applicable method for preparation of this ring system and will be discussed further in Sections... 

Stability of both the parent system and the S- oxide is conferred by the presence of electron-withdrawing substituents on the carbon framework, especially at positions 2, 4 and 6, while electron-donating substituents on sulfur also help. Compounds (48), (49) and (50) illustrate these conclusions, as they are air-stable, isolable species (74CL1101). Exactly analogous factors acting on stability are seen with acyclic sulfonium and sulfoxonium ylide compounds. 

Table 1 shows the two dihydrothiopyrans, and the two corresponding benzo fused systems. The monocycles (5,6-dihydro-2//- thiin and 3,4-dihydro-2iT-thiin) are quite clearly from their chemistry an allylic sulfide and an enol sulfide respectively, and in many of the reactions they exhibit they are perfectly comparable with acyclic counterparts. Once again, as for the tetrahydrothiopyrans, in many cases the principal chemical interest is related to the effects of preferred conformations on the stereochemistry of particular conversions, but this will not be discussed in great length as it is entirely predictable from classical alicyclic work. Where differences exist they may be attributed to the interaction of the heteroatom with the neighbouring alkene, which is not too considerable, and more importantly the reactivity of the sulfur atom in its own right. The benzo fused compounds, of course, have their own particular chemistry which has received considerable attention over many years, some of which will be discussed. 

Several recent reviews have included specific types of electrophilic cyclofunctionalization reactions.1 Important areas covered in these reviews are halolactonization u cyclofunctionalization of unsaturated hydroxy compounds to form tetrahydrofurans and tetrahydropyrans lb cyclofunctionalization of unsaturated amino compounds lc cyclofunctionalization of unsaturated sulfur and phosphorus compounds ld lf electrophilic heterocyclization of unconjugated dienes 1 synthesis of y-butyrolactones 1 h synthesis of functionalized dihydro- and tetrahydro-furans lj cyclofunctionalization using selenium reagents lk lm stereocontrol in synthesis of acyclic systems 1" stereoselectivity in cyclofunctionalizations lP and cyclofunctionalizations in the synthesis of a-methylenelactones.lq Previous reference works have also addressed this topic.2... 

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