Pyrans => conjugated aldehydes

Knoevenagel condensation of 1 2.1 stoichiometric ratio of the pyran and aldehyde generates a pure bis-condensation product. Due to the extended ir-conjugated system, these bis-condensed red emitters are about 40-50 nm red-shifted compared with their respective mono-substituted DCM analogues. 

Extensively conjugated aldehydes. The pyrylium salt reacts with CH2=PPh, at C-2. The resulting 2W-pyran-2-methylphosphonium salt is a precursor of the masked to-formylpentadienyl Wittig reagent. 

Mono-enolisation of a 1,5-diketone, then the formation of a cyclic hemiacetal, and its dehydration, produces 4//-pyrans, which require only hydride abstraction to arrive at the pyrylium oxidation level. The diketones are often prepared in situ by the reaction of an aldehyde with two moles of a ketone (compare Hantzsch synthesis, 8.14.1.2) or of a ketone with a previously prepared conjugated ketone - a chalcone in the case of aromatic ketones/aldehydes. It is the excess chalcone that serves as the hydride acceptor in this approach. 

The synthesis of the C1-C7 fragment, which corresponds to the lactone, starts with the homoallylic alcohol 2 which was prepared from 1. The existing stereocenter and the conjugate addition method of Evans [21] allow the control of the C5 stereocenter. The homoallylic alcohol 2 was oxidatively cleaved and homologated to the trans enoate 3 by a Wittig olefination. Treatment of 3 with benzaldehyde and a catalytic amount of KHMDS provided acetal 4. The internal Michael addition of the hemiacetal intermediate proceeds with complete stereoselectivity [22]. After deprotection and oxidation, the corresponding aldehyde was treated with Amberlyst-15 and then with camphor sulfonic acid (CSA), to yield pyrane 5 as a mixture of (3- and a-anomers (1.8/1). This compound was converted to the thiophenyl acetal 6 (4 steps) as this compound can be hydrolyzed later under mild conditions (Hg +) with subsequent oxidation of the lactol to the desired lactone. Compound 6 represents the C1-C7 fragment of discodermolide (Scheme 1). 

Destannylative reactions. l-Trimethylstannyl-2,4-pentadiene reacts with aldehydes and ketones in a -y-selective manner, whereas conjugated dienes are generated from 3-(tributylstannyl)propenyl pyrans. ... 

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