Diels-Alder reactions alcohol-catalysed

Finally the influence of the temperature and addition of ethanol on the enantioselectivity of the Diels-Alder reaction was studied. Table 3.3 summarises the results for different aqueous media. Apparently, changes in temperature as well as the presence of varying amounts of ethanol have only a modest influence on the selectivity of the Cu(tryptophan)-catalysed Diels-Alder reaction in aqueous solution. However, reaction times tend to increase significantly at lower temperatures. Also increasing the alcohol content induces an increase of the reaction times. 

In 2005, another class of chiral ligands, bis(thiazolines) derivatives, was prepared by Nishio et al. from chiral bis-(A-acylamino alcohols) by using Lawesson s reagent. These new compounds have proved to be useful chiral ligands for the Zn-catalysed Diels-Alder reaction of 3-acryloyloxazolidine-2-one with cyclopentadiene, giving the corresponding cycloadducts as a 94 6 diastereomeric mixture, where the major diastereomer was formed with 92% ee (Scheme 5.14). 

Lanthanide(ni) on ion exchange resins catalyse Mukaiyama aldol reactions in aqueous media, acetalisations, additions of silyl enol ethers to imines, saz-Diels-Alder reactions and the ringopening of epoxides with alcohols as depicted in Scheme 3.6.7. 

The dienophile may also be activated by hydrogen bonding of the carbonyl oxygen with suitable protic molecules and it has been observed that Diels-Alder reactions are accelerated when performed in protic solvents such as 2-butanol. Thus the opportunity exists for the development of an enantioselective alcohol-catalysed asymmetric Diels-Alder reaction. Indeed, Rawal and coworkers have recently discovered that the use of catalytic amounts of the enantiomerically pure diol a,a,a, a -tetraaryl-l,3-dioxolane-4,5-dimethanol (TADDOL) (8.113) in the Diels-Alder reaction of aminosilyloxydiene (8.114) with 2-substituted acroleins such as (8.24), results in the formation of the cycloadduct (8.115) with high ee. Simiharly, the strong Bronsted acid (8.116) effectively catalyses the Diels-Alder reaction of sUyloxydienes such as (8.117) with a,P-enone (8.105). ... 

An outstanding example of a branching pathway exploited complementary cyclisation reactions to yield alternative molecular scaffolds (Scheme 1.5). A four-component Petasis condensation reaction was used to assemble flexible cyclisation precursors e.g. 14). Alternative cyclisation reactions were then used to yield products with distinct molecular scaffolds Pd-catalysed cyclisation (- 15a) enyne metathesis (- 15b) Ru-catalysed cycloheptatriene formation (- 15c) Au-catalysed cyclisation of the alcohol onto the alkyne (- 15d) base-induced cyclisation (- 15e) Pauson-Khand reaction 15f) and Miesenheimer [ 2,3]-sigmatropic rearrangement (not shown). Four of these cyclisation reactions could be used again to convert the enyne 15e into molecules with four further scaffolds (17a-d). In addition, Diels Alder reactions with 4-methyl-l,2,4-triazoline-3,5-dione converted the dienes 15b, 17a and 17d... 

On the other hand, Kita et al. reported a combination of the domino reaction concept and the DKR protocol, comprising the first lipase-catalysed domino process that combined the DKR of alcohols by using l-etho QAanyl esters and the Diels-Alder reaction of the intermediates. The finding that ruthenium catalysts produced a rapid racemisation of the slow-reacting (S)-enantiomers was the key to the success of this process, which provided useful chiral intermediates for natural products, such as compactin and forskolin (Scheme 8.28). 

A stereoselective Claisen rearrangement occurs when butyro-ortholactones are heated with allylic alcohols the products are substituted butyrolactones. In contrast to the low dienophilicity that is usually observed for a -unsaturated lactones, the ester lactone (159) readily undergoes stannic-chloride-catalysed Diels-Alder reaction at room temperature. 3-Thiolen-2-one (160) is a useful... 

The route to pseudomonic acid A that has been outlined by Snider and Phillips is based on the Lewis-acid-catalysed ene reaction between the dienq (148) and formaldehyde, leading to the alcohol (150) by way of the quasi-intramolec-ular Diels-Alder reaction of the intermediate complex (149). The alcohol (150) was then converted into the intermediate (151), which had previously been converted into pseudomonic acid A (152) by Kozikowski and co-workers. ... 

S)-5-Hydroxymethyl-2(5H)-furanone (8), derived from D-ribono-lactone, was used as an intermediate in a synthesis of the 6a-carbocycline analogue (9), with a Pauson-Khand reaction being used to build up the ring system from enyne (10) (Scheme 4). When alcohol (8) underwent Diels-Alder reaction with cyclopentadiene, either the (2S, 3R)-isomer (11) or its enantiomer could be produced by appropriate manipulation of the initial adduct face-selective Lewis-acid-catalysed Diels-Alder reactions of a,iS-unsaturated esters derived from D-mannitol could be used to produce the (R,R)-isomer (12) or its enantiomer.This chemistry was extended to give syntheses of (+)-0-santalene (13) and its enantiomer from (8) the starting material was prepared as described in Vol. 20 p.260, but a number of other papers dealing with routes to this intermediate are mentioned in the next section. 

The source of alcohol 50 is most probably acid-catalysed hydrolysis of 49 to the nitrosocarbonylbenzene intermediates 51, which, like acid chlorides, react with water to give benzoic acids 52 (Scheme 1 pathway (i)).159 Acylnitroso intermediates 51 were trapped as the Diels Alder adducts 53 in reactions in CH3CN/H20 and in the presence of cyclopentadiene. In CH3CN/10% H280, 53 was enriched in lsO... 

The Y zeolites have been identified as active catalysts for various reactions such as isomerisation, amination, alkylation and deamination [1-4]. Cu containing Y zeolites have been found particularly useful to catalyse Diels - Alder cycloaddition [5-6]. Dehydration of t-BuOH has been suggested as a model reaction for estimating the zeolite acidity [7]. The dehydration of t-BuOH has received particular attention as tertiary species gives the most stable carbonium ion. The dehydration of alcohols by zeolites has been extensively studied... 

From Achiral Non-carbohydrates. — 3-Deoxy-3-guanidino-D-threose 48 equilibrates with 49. a transition state inhibitor for galactosidase. It was synthesized as shown in Scheme 12 from epoxide 47, which was obtained by porcine pancreatic lipase catalysed enantioselective esterification of the racemic epoxy-alcohol precursor. 6-Deoxy-L-talonolactone 50 was synthesized by an asymmetric aldol condensation - dihydroxylation sequence (Vol.24, p.lS2) in improved diastereoselectivity and was converted into 2-acetamido-2,6-dideoxy-L-fucose (shown as its furanose isomer 51 in Scheme 13), 3-acetamido-3,6Hlideoxy-L-idose and 5-acetamido-S,6-dideoxy-D-allose by S 2 displacements of triflate with azide ion. 4-Amino-4-deoxy-DL-erthrose 53 was obtained from the hetero-Diels-Alder adduct 52 by a sequence of reactions including cis-dihydroxylation (OSO4, NMNO) of the alkene moiety (Scheme 14). The synthesis of a racemic branched-chain lactam is covered in Chapter 16. 

Michael/Henry/dehydration/aromatisation reaction of 2-(2-oxoethyl) benzal-dehydes and nitroalkenes mediated by pyrrolidine to obtain polysubstituted naphthalene derivatives. DBU catalysed the conjugate additions of alcohols to ot,p-unsaturated nitriles, esters and ketones. Perhaps more important are the aza-Michael addition reactions of amines to a,p-unsaturated ketones, nitriles and esters. Recently, Costa, Vilarrasa and coworkers described the addition of lactams, imides, 2-pyridone, pyrimidine-2,4-diones and inosines to methyl propiolate and other similar compounds, DABCO and DMAP being the best catalysts. As mentioned before, tertiary amines give zwitterionic species with activated allynes. It was as early as 1932 when Diels and Alder used the reaction of pyridine with dimethyl acetylenedicarbojylate (DMAD) for the synthesis of heterocycles. The interception of the corresponding intermediate with Al-tosylimines and activated olefins provided access to l-azadienes ° and highly substituted butadienes (Scheme 2.6). When the quenching species of the zwitterionic intermediate is a 1,2-diketone, dibenzoyl maleates or cyclopentenedione derivatives could be obtained (Scheme 2.6). The interception of the zwitterionic species of N-methyl imidazole (NMI) and DMAD with ketenes to obtain unsaturated esters has also been shown. ... 

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