in black and white
Main menu
Home About us Share a book
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics

Sulfur reagents in organic - Metzner P.

Metzner P., Thuiller A. Sulfur reagents in organic - Academic press, 1994. - 200 p.
Download (direct link): sulfarreagents1994.djvu
Previous << 1 .. 48 49 50 51 52 53 < 54 > 55 56 57 58 59 60 .. 67 >> Next

160C. 7 d
- 70-80%
R = D. Me. prenyl
The experimental conditions indicated in the previous scheme (7 d at 160C) reflect the general observation that substituted vinyl sulfones are only moderately reactive dienophiles. Again, the introduction of a second electron withdrawing group enhances their reactivity and extends the scope of dienes which can be used as partners. Compounds of the type ArS02CH-CHX (X = N02, COOR, CF3 or S02Ar) have been particularly investigated.
Cycloadducts with great synthetic potentialities have been easily obtained from the reactions of dienes with allenic sulfones (ArS02CH=C=CH2) and acetylenic sulfones (essentially ArS02C=CH, but some disubstituted acetylenes (R'SOjC^C-R2) have been used). A few applications are briefly summarized in the following schemes.
2-Phenylnorbomadiene [534]:

+ PhS02-C= C-Ph
1-Azanorbornadiene [535]:
^ COaMe
42% 71%
A Diels-Alder reaction of N-(carbomethoxy)pyrrole and phenylsulfonyl
6-chloro-3-pyridyl acetylene was used similarly in a total synthesis of epibatidine, an alkaloid heralded for its high analgesic potency [536].
(m.p. 56-57 C)
An intramolecular Diels-Alder reaction of an alkynyl sulfone was used [537] in the synthesis of a ring fragment of vitamin D3.
4.5.5 Cheletropic processes
Since the first report in 1914 of the formation of a butadiene-sulfur dioxide adduct [538], much work has been carried out on the reaction of conjugated dienes with sulfur dioxide and its applications in synthetic strategies. Two pathways can and have been observed: a cheletropic reaction (w 2s + 4s) yielding 3-sulfolenes and (4 tts + 2s) hetero-Diels-Alder addition yielding sultines (see [539] and [540] and references
therein for this latter process). Only the much more usual sulfolene formation is often to be taken into consideration. For instance, with isoprene the formation of a sultine is only observed at low temperature under catalysis (CF3COOH or BF3-Et20), and above -40C the sulfolene is formed [539].
The thermal retro-reaction of 3-sulfolenes to dienes and sulfur dioxide occurs under mild conditions (about 120-200C), and is, as predicted from the Woodward-Hoffmann rules, a disrotatory process; so for 2,5-dimethyl-
3-sulfolenes [541,542]:
Many uses of the sulfolene reaction for synthetic purposes are associated with this easy stereoselective synthesis of conjugated dienes [109, 203]. The development of this methodology led to new routes for the introduction of electrophiles in the 2 and (or) 5 positions of the sulfolenes
as well as for the extrusion of sulfur dioxide. For this latter process, although the thermolytic method is most generally favoured, reactions of the 3-sulfolenes with LiAlH4 (543] or with ultrasonically dispersed potassium (UDP) [544] were found to be much useful [545]. In the presence of t-BuOH as a proton source at room temperature, the UDP-induced S02 extrusion from 2,2,5,5-tetrasubstituted-3-sulfolenes was particularly convenient [545], whereas the thermal reaction occurred only at about 350C in such cases.
3,6-Diethyl-3,5-octadiene from a sulfolene
To a suspension of UDP (4.5 mmol) in toluene (10 ml) in a laboratory ultrasonic cleaning bath under N2 was added a solution of 2,2,5,5-tetraethyl-3-sulfolene (1.5mmol) in toluene (10ml). The sonication was continued for 30 min, during which time a bright-blue colour developed. A solution of t-BuOH in THF (0.45 mol dm~3, 4.4 mmol) was added dropwise over a period of 30 min, whereupon the potassium was completely consumed. The mixture was filtered through a short silica gel column to remove the solid precipitate, and the filtrate was concentrated under reduced pressure. Purification by HPLC (Lichrosorb column, hexane) yielded the corresponding diene in a 92% yield.
From [545] with permission.
The synthesis of (8?,10?)-8,10-dodecadien-l-ol [546], the sex pheromone of the codling moth (Laspeyresia pomonella), illustrates the way followed by Bloch et al. to introduce stereoselectively substituents a and a' to the sulfone group of a potential 3-sulfolene via a retro-Diels-Alder reaction. The starting sulfone (1) is readily available and, at the temperature required for the retro-Diels-Alder process, cleavage of the sulfolene occurs, yielding the (E, ?)-conjugated diene (2) and, more generally, dienes (3) in high yields and stereoisomeric purities [546].
Takayama et al. [547] have shown that 3-sulfolene can be directly alkylated at the 2 and 5 positions. The 2-substituted 3-sulfolenes (4) were obtained in a fair yield when the labile sulfolene a-carbanion was generated by LHMDS in a THF-HMPA solution at -78C in the presence of an alkyl iodide. /ran.s-2,5-Dialkyl-3-sulfolenes (5) were similarly
i) UDP
11) t-BuOH
Previous << 1 .. 48 49 50 51 52 53 < 54 > 55 56 57 58 59 60 .. 67 >> Next