On the ozonolysis of unsaturated tosylhydrazones as a direct approach to diazocarbonyl compounds

Reactive uptake of ozone to azo dyes in a coated-wall flow tube

Azo dyes are the most common colorants in consumer products, including clothing and cosmetics. Some azo dyes and their products from reductive degradation are known to be mutagenic, so dermal exposure to these species has been studied extensively. In contrast, oxidative degradation of azo dyes in consumer products has not been studied so thoroughly. In the indoor environment, ozone is ubiquitous, so reactive uptake of ozone to azo dyes could lead to dermal exposure to other classes of degradation products. Here, we report the first measurements of the reactive uptake of ozone to thin films of three widely used commercial azo dyes: sunset yellow, amaranth, and tartrazine. Steady-state uptake was observed for all three dyes, under all conditions investigated, even at the lowest relative humidity (RH) of 0%. The uptake coefficients increased with RH. For sunset yellow at 100 ppb of ozone, the value at 80% RH, (2.0 ± 0.5) × 10^-7, was 2.5 times greater than that at 0% RH, (8 ± 1) × 10^-8, consistent with plasticization of the thin film due to absorption of water. The uptake coefficient of sunset yellow at 80% RH exhibited an inverse dependence on the ozone mixing ratio, approaching an asymptote of 1 × 10^-7 above 250 ppb. At 80% RH and 100 ppb of ozone, the uptake coefficients for the three dyes were similar, (2.0 ± 0.5) × 10^-7 for sunset yellow, (2.7 ± 0.6) × 10^-7 for amaranth, and (3.2 ± 0.3) × 10^-7 for tartrazine, despite differences in structural parameters related to the number of reactive sites at the surface. Together, these results are consistent with ozone diffusing into the thin film and the dye molecules mixing between the layers, such that reaction is not restricted to the surface of the film. Finally, the results are suggestive of a role for azo dyes, including the occurrence of their oxidation products, in indoor chemistry.

Evolution of a Cycloaddition–Rearrangement Approach to the Squalestatins: A Quarter-Century Odyssey

The highs, lows, and diversions of a journey leading to two syntheses of 6,7-dideoxysqualestatin H5 is described. Both syntheses relied on highly diastereoselective n-alkylations of a tartrate acetonide enolate and subsequent oxidation–hydrolysis to provide an asymmetric entry to β-hydroxy-α-ketoester motifs. The latter were differentially elaborated to diazoketones which underwent stereo- and regioselective Rh(II)-catalysed cyclic carbonyl ylide formation–cycloaddition and then acid-catalysed transketalisation to generate the 2,8-dioxabicyclo[3.2.1]octane core of the squalestatins/zaragozic acids at the correct tricarboxylate oxidation level. The unsaturated side chain was either protected with a bromide substituent during the transketalisation or introduced afterwards by a stereoretentive Ni-catalyzed Csp3–Csp2 cross-electrophile coupling.

1 Introduction  

2 Racemic Model Studies to the Squalestatin/Zaragozic Acid Core

3 Asymmetric Model Studies to a Keto α-Diazoester

3.1 Dialkyl Squarate Desymmetrisation

3.2 Tartrate Alkylation

3.2.1 Further Studies on Seebach’s Alkylation Chemistry 

4 Failure at the Penultimate Step to DDSQ 

5 Second-Generation Approach to DDSQ: A Bromide Substituent Strategy 

5.1 Stereoselective Routes to E-Alkenyl Halides via β-Oxido Phosphonium Ylides 

5.2 Back to DDSQ Synthesis

6 An Alternative Strategy to DDSQ: By Cross-Electrophile Coupling

7 Alkene Ozonolysis in the Presence of Diazo Functionality: Accessing α-Ketoester Intermediates

8 Summary

Diastereoselective Synthesis of Pyrazolines by Metal-Free Rearrangement of Bicyclic Triazolines

The metal-free preparation of diazoalkanes through the ring rearrangement of bicyclic triazolines is reported here. Their use in 1,3-dipolar cycloaddition reactions with electron-withdrawing alkenes was investigated. This synthetic procedure allows differently substituted pyrazolines to be obtained in few steps and with high atom economy.

Application of The Dess-Martin Oxidation in Total Synthesis of Natural Products

Dess-Martin periodinane (DMP), a commercially available chemical, is frequently utilized as a mild oxidative agent for the selective oxidation of primary and secondary alcohols to their corresponding aldehydes and ketones, respectively. DMP shows several merits over other common oxidative agents such as chromiumand DMSO-based oxidants; thus, it is habitually employed in the total synthesis of natural products. In this review, we try to underscore the applications of DMP as an effective oxidant in an appropriate step (steps) in the multi-step total synthesis of natural products.

Alkylation of lithiated dimethyl tartrate acetonide with unactivated alkyl halides and application to an asymmetric synthesis of the 2,8-dioxabicyclo[3.2.1]octane core of squalestatins/zaragozic acids

(R,R)-Dimethyl tartrate acetonide 7 in THF/HMPA undergoes deprotonation with LDA and reaction at −78 °C during 12–72 h with a range of alkyl halides, including non-activated substrates, to give single diastereomers (at the acetonide) of monoalkylated tartrates 17, 24, 33a–f, 38a,b, 41 of R,R-configuration, i.e., a stereoretentive process (13–78% yields). Separable trans-dialkylated tartrates 34a–f can be co-produced in small amounts (9–14%) under these conditions, and likely arise from the achiral dienolate 36 of tartrate 7. Enolate oxidation and acetonide removal from γ-silyloxyalkyl iodide-derived alkylated tartrates 17 and 24 give ketones 21 and 26 and then Bamford–Stevens-derived diazoesters 23 and 27, respectively. Only triethylsilyl-protected diazoester 27 proved viable to deliver a diazoketone 28. The latter underwent stereoselective carbonyl ylide formation–cycloaddition with methyl glyoxylate and acid-catalysed rearrangement of the resulting cycloadduct 29, to give the 3,4,5-tricarboxylate-2,8-dioxabicyclo[3.2.1]octane core 31 of squalestatins/zaragozic acids. Furthermore, monoalkylated tartrates 33a,d,f, and 38a on reaction with NaOMe in MeOH at reflux favour (≈75:25) the cis-diester epimers epi-33a,d,f and epi-38a (54–67% isolated yields), possessing the R,S-configuration found in several monoalkylated tartaric acid motif-containing natural products.

Alkene protection against acid using a bromide substituent: application in a total synthesis of (−)-6,7-dideoxysqualestatin H5

Alkenyl bromide functionality survives several synthetic steps, including an acid-induced rearrangement, before a stereoretentive methylative debromination gives the natural product.