Tetraenes, epoxidation

The thermal isomerization of cyclooct tetraene epoxide hoe been studied at 260° by Btichi and Burgess,2012 who observed the formation uf threo ieomerio aldehydoa (Eq. 460a), one of which may be the elusive compound norcaradieno carboxaldehyde. 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

The tetraene precursor 14, assembled in a similar way to 11, underwent smooth cyclization using the ruthenium initiator 3 (0.1 equiv) to give macrolactone 15, again in good yield and with complete E-selectivity. Despite the incorrect olefin geometry, transformation into epoxides 16 provided further encour-... 

The range of the asymmetric epoxidation reaction may be extended still further to include dienes (Entries 7,12,17) and even tetraenes (Entry 26). It is of interest to note that only double bonds adjacent to the carbonyl function are epoxidised and any remaining double bonds are left untouched (Entry 26). This selective reactivity allows for further elaboration of unreacted alkene units at a later stage, (see Sect. 5). Enediones (Entries 21-23) and unsatuxated keto esters (Entries 24 and 25) can also be oxidised in good yields and good to excellent stereoselectivity using polyamino acids. 

Disubstituted dihydropyrans are produced with high u/iri-selectivity when 2-phenyl-4-(4-tolylsulfonyl)-3,4-dihydro-2H -pyrans ate treated with Al-based Lewis acids <99SL132>. Tetraenes 10, derived from dienes via their epoxides, undergo a double RCM reaction under Ru-catalysis to yield polycyclic ethers 11 in which the dihydropyran units can be joined by a variable number of carbon atoms <99JOC3354>. Continued work on the use of dispiroketals in synthesis has led to an improved route to the enantiomers of bi(dihydropyrans) 12 <99JCS(P1)1639>. 

Similarly, acetone sensitized irradiation of the epoxy tetraene 13 resulted in ring closure to the caged epoxide M.171... 

Vinylcyclopentenones. 1,2,4,6-Tetraenes are converted into 4-vinylcyclopen-tenones by epoxidation under the mild conditions of Anderson and Veysoglu (5,120).1 Acetoxymercuration is not useful for this cyclization. 

Bromination of 3,10-epoxycyclo[10.2.2.02,11.04 9]hexadeca-4,6,8,13-tetraene (11) gave (12) and several by-products. Acid cleavage of corresponding epoxide proceeded in a similar fashion.27... 

Syntheses of 16,17-disubstituted oestra-l,3 5(10),14-tetraene 3-methyl ethers involved reaction of the corresponding 16,17-epoxides with various nucleo-philes and similar reactions of the 17,20-epoxides (300) gave a range of androstanes (301). A series of 16j8-alkylated androstanes and oestranes has... 

Several groups have investigated the use of complex sulfur ylide reagents for a key carbon-carbon bond formation step in leukotriene synthesis (equation 4). For example, tetraene (6) reacted with methyl S-oxopentanoate to give an 8S% yield of the desired epoxides as a mixture of cis and trans isomers. Significantly, only small amounts of by-products resulting from elimination or rearrangement of the sulfonium salt were observed. 

The natamycin structure consists of a 26-membered lactone ring containing a tetraene chromophore, an a. unsatu-rated lactone carbonyl group, three hydroxyl groups, a carboxyl group, a traits epoxide, and a glycosidically joined mycosumine. Like the other polyene antibiotics, natamycin is amphoteric. 

Because of the success with iron cyclam, our search for other non-heme iron complexes involved a survey of cyclam like ligands including HMCD (5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazocyclodec-4,l 1-diene), TIM (2,4,9,10-tetramethyl-l,4,8,ll-tetraazocyclodec-l,3,8,ll-tetraene), TDO (1,4,8,ll-tetraazocyclodecane-5,7-dione), etc. Under the same reaction conditions as for iron cyclam, the iron complexes of these other ligands also proved to be efficient catalysts for olefin epoxidation with PhIO and MCPBA, but not for H2O2. These results further support the notion that the three oxidants behave differently in their reactions with these non-heme iron complexes. 

Reaction of 2,3,5-tri-(9-benzoyl-D-ribose with a Wittig reagent gave access to the unsaturated aldonic acid derivative (25) from which the epoxide (26) was obtained. This was then used to prepare the tetraene (27) and hence (28), which is the slow reacting substance of anaphylaxis. ... 

In 2000, in an effort to verify the structure of a polycyclic oxasqualenoid, glabrescol 71, Corey and co-workers applied the Shi epoxidation in the conversion of tetraene 69 to tetra-epoxide 70, which was subsequently transformed to glabrescol 71 in three steps/ ... 

Lipoxins constitute an additional family of bioactive LOX products which are characterized by a conjugated tetraene structure and three asymmetric carbons to which hydroxy groups are attached [25]. Lipoxins are abbreviated to LX followed by a capital letter (A or B) that indicates the position of the OH-groups, and by a subscript which gives the number of the double bonds. The biosynthesis of lipoxins [26,27] involves a concerted action of 5- and 15-LOXs or of 5- and 12-LOXs as well as an enzymic hydrolysis of an epoxide intermediate (Scheme 6). However, in vitro experiments with purified rabbit 15-LOX suggested that lipoxin B4 can be formed... 

Corey and coworkers reported that (i )-2,3-dihydroxy-2,3-dihydrosqualene (104) was rapidly converted into proposed glabrescol 90 in 31% overall yield by enantio-selective pentaepoxidation with ketone 42 and subsequent cascade epoxide opening (Scheme 3.34) [69]. In their efforts to determine the correct stmcture of glabrescol, tetraene 106 was epoxidized with ketone 42. The resulting epoxide 107 was transformed into a chiral C2-symmetric pentacydic oxasqualenoid 109 (Scheme 3.35), which matches the reported isolated natural product glabrescol [70, 71]. 

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