Two-fold extrusion

4-Thiadiazolines were easily prepared from ketones or thioketones. By pyrolysis in the presence of a phosphine they afforded olefins [128,441] (see Section 4.5.4). 

Extrusion of carbon dioxide and sulfur was also used by Barton [441], as in the following example  

These methods were mainly used in the synthesis of crowded tetrasubstituted alkenes (in search of tetra(t-butyl)ethylene). 

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An extensive discussion and leading references on the stereochemistry of thiadiazolines and their pyrolysis to alkenes by a two-fold extrusion process can be found in (78JCS(Pl)45> (see also Section 4.27.4, Scheme 33). 

A convenient synthesis of sulphenyl chlorides has been developed in which a thiol is reacted with chlorocarbonylsulphenyl chloride and the resulting adduct desulphurized by triphenylphosphine. Further examples - have been enumerated in which olefins are stereospecifically synthesized by a two-fold extrusion process, one of which is the removal of sulphur by a phosphine, e.g. in the synthesis of bicyclobutylidene (25). 

The full paper on the preparation and properties of monomeric selenoketones and their conversion into olefins by two-fold extrusion processes has now appeared. The preparation of the spiro-ketones (459)—(461) from 7-norbornen-one has been described. Thus, treatment of 7-norbornenone with cyclopropyl-... 

Olefin synthesis by two-fold extrusion of nitrogen and sulfur from a A -l,3,4-thiadiazoline intermediate. Particularly applicable to the synthesis of moderately hindered tetra-substituted ethylenes ... 

With a more complex diazo compound containing amino and ester groups, products resulting from 1,3- and 1,5-dipolar elec-trocyclization are obtained. Two-fold extrusion reactions with 1 and diverse diazo compounds are reported to be aimed at the preparation of stericaUy crowded ethylenes. An example with bis(/er/-butyl)diazomethane (25) is shown in eq 12. 

Alkene Synthesis.—The alkene synthesis by two-fold extrusion processes (see Vol. 3, pp. 246, 693) is discussed in Chapter 2 of this Volume (p. 89). The addition of diazo-compounds to suitable thiones provides an alternative route to A -l,3,4-thiadiazolines, which give episulphides on pyrolysis and hence alkenes on further reaction with triphenylphosphine. The symmetrical 1,3,4-thia-diazoline represents the normal direction of cycloaddition, but the orientation of addition of diazomethane to adamantanethione is governed by the nature of the solvent, and the unsymmetrical adduct predominates in solvents of higher polarity. A -l,3,4-Thiadiazoline-2,5-dione (148) is generated by oxidation of... 

A -l,3,4-Thiadiazolines.—A novel synthesis of highly hindered olefins is based on the principle of a two-fold extrusion process (Scheme 8). ... 

Use of unmodified starches in combination with mild acids and carbonates has been claimed.214 Starch moisture content should be no greater than 25%. Acids such as tartaric, citric or malic acids are added at levels between 0.2% and 7%, while the carbonate level is 0.1% to 2%, both based on total starch composition. The acid is stated to serve a two-fold function it depolymerizes the starch during extrusion via hydrolysis, improving expansion at the same time, it liberates carbon dioxide, which acts as a blowing agent, from the carbonate. A typical formula extruded in a twin-screw extruder at 170-195°C yielded a product with a density of approximately 1 lb/ft3 (16kg/m3) and resilience of 60-85%. The foam had a continuous skin with a closed cell structure. 

The major cation present In fruits and vegetables Is K. Its concentration can be affected by environmental factors, principally the amount of available E In the soils or root media f 16, Different applications of E resulted In more than a two-fold difference In snap bean pod E (JD, Table III. Higher pod E levels were associated with softer beans when measured after a standard canning procedure. Firmness values In this table and In Tables IV-V1 and In Figures 1 and 2 were obtained as kg force resistance to compression In a back extrusion coll (Jl). 

The two-stage model is conceptually very useful because it separates the thermodynamics of insertion from helix assembly. When examining the stability of membrane proteins in the bilayer, the second stage is presumably the most relevant since the extrusion of the protein back into the aqueous environment should be extremely unfavorable. This idea is supported by the experiments discussed below. The two-stage model may not contain sufficient detail to describe the folding process in all cases, however. For example, the F and G helices of bacteriorhodopsin do not spontaneously form helices in vesicles, indicating that these peptides require assistance from the remainder of the protein to fold properly (Hunt et al., 1997). 

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