Selenoaldehydes Diels-Alder reactions

Segi and coworkers have invent a direct method for converting aldehydes to selenoaldehydes. This procedure can be used to effect intermolecular reactions as well as intramolecular selenoaldehyde Diels-Alder reactions as outlined in equation (108). 

On transforming the cyclopentenone 1431 into the trimethylsilyloxy diene 1432 the ensuing Diels-Alder cyclization gives rise to 69% tricychc compound 1433 [10] (Scheme 9.8). For Diels-Alder-reactions of thioaldehydes, selenoaldehydes, or unsaturated nitroso compounds with cyclopentadiene, see the reactions of 602 to 603 and of 605 to 606 in Scheme 5.48 and of 1092 to 1093 in Scheme 7.43. For Diels-Alder-reactions of silyloxyazadienes such as 510 with maleic anhydride to give 511, see Scheme 5.29. 

When cyclic 1,3-dienes were employed, the reactions were found to be exo selective [Eq. (28)]. In contrast, the Diels-Alder reactions of free thio-and selenoaldehydes with cyclopentadiene occur with a preference for the endo isomer.235 Thus, coordination of thio- and selenoaldehydes to a penta-carbonylmetal fragment resulted in a reversal of the endo selectivity. The exo/endo product ratio of [W(CO)5 E = C(Ph)H ] was 7.3 1 (E = S) and 2.6 1 (E = Se), whereas the kinetically controlled exo/endo ratio of the cycloadducts obtained by trapping of the free heterobenzaldehyde with cyclopentadiene was reported to be 1 7236 and 1 4235 237 (E = S) and 1 2.6238 and 1 4239 (E = Se), respectively. 

Although a,/8-unsaturated thio- and selenoaldehyde complexes also seemed to behave as dienophiles in Diels-Alder reactions with cyclopentadiene, the products were too unstable for characterization. Therefore, it remains unknown which double bond (E=C or C=C) was involved in the cycloaddition.64... 

Sakurai et al. were successful in reacting even tetramethylsilene (Me2Si=SiMe2) with benzene in a photochemical [2 + 4] cycloaddition at 10 K in an argon matrix [472]. With mentioning the suitability of iminoboranes [473] and selenoaldehydes [474] to serve as dienophiles in hetero Diels-Alder reactions, this enumeration of exemplary, less usual hetero Diels-Alder reactions shall be completed. 

The selenoaldehydes and selenoketones generated in situ by a wide range of methods can generally be trapped with dienes [30, 31, 56, 57, 60-63]. Cyclo-pentadiene has often been used, and the hetero-Diels-Alder reaction proceeds to give endo-adducts as major products. The regioselectivity of the reaction... 

Early attempts to prepare selenoaldehydes by treatment of aldehydes with H2Se under acidic conditions yielded oligomeric or polymeric sele-nides, possibly via self-condensation of selenoaldehydes [15JPR116 67JCS(B)117]. In 1988, Japanese workers reported that the reaction of aldehydes with bis(trimethylsilyl)selenide in the presence of a catalytic quantity of butyl lithium afforded selenoaldehydes (88JA1976). This novel method has been exploited for the first intramolecular Diels-Alder reactions of selenoaldehydes (88TL6965). Dienal (79) was converted into dienselenal (80) and then into a mixture of stereoisomeric adducts 81 and 82. The major product was as-fused isomer 81, derived from the endo transition state. These results compare favorably with intramolecular thio-aldehyde reactions (Section II,B). 

Anodier selenoaldehyde preparation has been develqied by Kirby and Trethewey (equation 107). Dienophile (199) can be trapped directly by 1,3-dienes or by anthracene to give adduct (200). Retro Diels-Alder reaction of (200) in the presence of another diene affords a new cycloadduct. 

A mechanistic proposal includes addition of MesSiSeLi to aldehydes, followed by elimination of MesSiOLi to generate selenoaldehydes, which undergo Diels-Alder reaction with dienes. Aldehydes bearing a conjugate diene unit provide intramolecular Diels-Alder adducts (eq 7). ... 

Free intermediate thioaldehydes 598 or 602 and the selenoaldehydes 605 and HMDSO 7 are obtained in THF at 0°C on treatment of aliphatic and aromatic aldehydes with bis(trimethylsilyl)thiane 601 or bis(trimethylsilyl)selenide 604 in the presence of traces of butyllithium, while trapping the sensitive intermediate thio- or selenoaldehydes 602 and 605 with cyclopentadiene or cyclohexadiene to furnish mixtures of endo and exo Diels-Alder adducts such as 603 a and 606 a and 603 b and 603 b [148-150], the exo/endo ratio of which can be controlled [150] (Scheme 5.48). Analogous reaction of ketones such as 2-adamantanone or acetylene ketones with MesSiXSiMes 608 (a. X=S (601) b. X=Se (604)) in the presence of... 

Selenoaldehydes.1 The first step in a synthesis of selenoaldehydes involves addition of dimethylphenylsilyllithium2 to aldehydes followed by trapping of the adduct with TsCl. The a-silyl tosylates are converted into a-silyl selenocyanates by reaction with KSeCN catalyzed by 18-crown-6. These products are easily purified, and are converted by fluoride ion into selenoaldehydes, which can be trapped by Diels-Alder cycloadditions (equation I). 

An efficient 1,3-dipolar cycloaddition has been developed to prepare 2,3-dihydro-[l,3,4]selenadiazoles 233 <07TL2302>. The reaction involves selenoaldehyde-anthracene adduct 230 and hydrozonyl chloride 231 in refluxing toluene. The [2+3] cycloaddition of reactive selenoaldehydes 229 (generated from 230 via thermal retro Diels -Alder) and nitrile imine 232 (created via dehydrochlorination of 231 with triethylamine) allows the formation of dihydro-[l,3,4]selenadiazole 233 as a single regiomer. 

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