1.4- Dithiins, 2,3-dihydro- from

Vinyl-3,6-dihydro-l,2-dithiin 2-oxide has been isolated as one of the main components from garlic Allium sativum) <2001MI867>. Its structure was elucidated by NMR and MS. 

Only a few examples of ring syntheses by transformation of another ring have been published, as exemplified by the preparation of the sultines 168 by ring enlargement of five-membeted thiolane 1-oxides 166 (cf Section 8.10.9.2.3) and the 3,6-dihydro-l,2-dithiins 202 by catalytic transformation of vinylthiiranes 201 (cf Section 8.10.9.4.2). Because possibilities to synthesize six-membered rings with oxygen and/or sulfur as heteroatoms in 1,2-positions are rather limited, these reactions have been covered already in Section 8.10.9 together with alternative syntheses from alicyclic compounds. 

Synthesis of 2,3-dihydro-l,4-dithiin 11 was accomplished from l,3-dithiol-2-one 247 in the presence of dibro-moethane and potassium hydroxide <1998JOG3952>, while reaction of 2,3-dichloro-l,4-dioxane with powdered Zn in hexamethylphosphoramide (HMPA) was used for the synthesis of 1,4-dioxene 10 <1998JPP10067773>. To obtain substituted 1,4-oxathianes, the hydrogenation of the corresponding partially saturated compounds has been employed <2001J(P1)2604>. 

The stereospecific formation of 5,6-dihydro-l,4-dithiins from the reaction of the 13-dithiete 60 with alkenes has been shown to proceed through its valence isomer, l,2-bis(methoxycarbonyl)ethane-13-dithione (Scheme 42) <99JOC8489>. 

Dihydro-1,4-oxathiins (336 Z = 0) and 5,6-dihydro-1,4-dithiins (336 Z = S) are easily obtained from 1,3-oxathiolanes (335 Z = 0) and 1,3-dithiolanes (335 Z=S), respectively, by treatment with bromine (9IS223), A-bromosuccinimide (94T7265), chlorine (87JOC5374, 91S223), or sulfuryl chloride (88JCR(M)1401>. 

The monobenzo-fused derivatives of 1,4-dioxin, 1,4-oxathiin and 1,4-dithiin, (345), (346) and (347), can all be prepared by base-catalyzed reaction between the appropriate 1,2-disubstituted benzene and an a-haloketal via an intermediate 2-alkoxy-2,3-dihydro derivative (348). The pyrolysis of the acetoxy derivative (349) at 450°C gives (345 80%) (67ZC152). 2-Hydroxy-2-phenyl-l,4-benzodioxane, from catechol and phenacyl bromide, is dehydrated to (345) by thionyl chloride in pyridine. 

An excellent method for the preparation of gem-difluoro compounds from aldehydes and ketones consists of conversion of the carbonyl compound to the corresponding 1,3-dithiolane followed by treatment with two equivalents of l,3-dibromo-5,5-dimethylhydantoin (DBH) and pyridinium poly(hydrogen fluoride) (HF-pyridine) in methylene chloride. Attempted extension of this procedure to 7-methoxy-2,2-dimethyl-4-chromanone, however, gave only the dihydro- 1,4-dithiin derivative 1 in 78% yield. This transformation, which proceeded in excellent yield with a variety of 4-chromanones, was found to require only the DBH (i.e. fluoride ion played no role). 

Dioxins, 1,4-oxathiins, and 1,4-dithiins are commonly prepared by elimination reactions from saturated analogues (see Section 4.3.4.1.4). A convenient synthesis of 2,3-dihydro-1,4-dioxins (e.g., 442) starts from propargyl chloride and 1,2-ethanediol (Scheme 206) . Another approach to substituted 2,3-dihydro-... 

Dithiolanes derived from chromanones do not form the ew-difluoro compounds, instead they undergo l,3-dithiolane-dihydro-1,4-dithiin rearrangement giving 10. ... 

Table 2 Synthesis of 3,6-dihydro-1,2-dithiins from vinylthiiranes...

The formation of naphthothiete (34) from dithiol 41 and naphthothiadiazine (42) is preceded by biradical intermediate 43 as proven by the photolysis of naphthothiadiazine (42) in carbon disulfide. After 15 min irradiation alongside naphthothiete (34) obtained in a 52% yield there also formed in a 22% yield naphtho[l,8-<7e]-2,4-dihydro-l,3-dithiin-2-thione (44) resulting from the reaction of biradical 43 with carbon disulfide (Scheme 7). 

An alternative synthesis of 2,3-dihydro-1,4-dithiin employs an unsaturated dithiolate ion. Thus, c/5-l,2-ethylenedithiolate, generated from 1,2-dichloroethylene as shown, reacts with 1,2-dibromo-ethane to give the desired compound (Scheme 7) <91SM2093). 

Dithiins can also be derived from the decomposition of other heterocyclic systems. Thus, the reactions of benzopentathiepin with norbornene, norbornadiene, and acrylonitrile in the presence of triethylamine alTorded the corresponding 2,3-dihydro-1,4-benzodithiins, in satisfactory yields <90SUL233>. The reaction of 2,3-dimethyl-2-butene with benzopentathiepin in the presence of boron trifluoride etherate give 2,2,3,3-tetramethyl-2,3-dihydro-l,4-benzodithiin in 89% yield <89CL2lll>. 

Dihydroxynaphthalene and 9,10-diacetoxyphenanthrene react with 1,2-diols and 1,2-dithiols in a one-pot synthesis of annulated 2,3-dihydro-1,4-dioxins and -1,4-dithiins (Scheme 26) <04TL1343>. The reaction of 2,3-dihydroxynaphthalene with 1,2-dihalogenated aromatic compounds leads to linearly annulated dioxins of particular interest are tri- and tetra-dioxins and various hetero-fused dioxins e.g. 62 (34%). Several examples yield cation radical salts on electrocrystallisation <04T8899>. Linear arrays of fused pyran-dioxin-cyclohexane rings as found in natural products derived from the milkweed family have been described e.g. 63 <04EJO4911>. 

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