Microviscosity is a key parameter controlling the rate of diffusion and reactions on the microscale. One of the most convenient tools for measuring microviscosity is by fluorescent viscosity sensors termed 'molecular rotors'. BODIPY-based molecular rotors in particular proved extremely useful in combination with fluorescence lifetime imaging microscopy, for providing quantitative viscosity maps of living cells as well as measuring dynamic changes in viscosity over time. In this work, we investigate several new BODIPY-based molecular rotors with the aim of improving on the current viscosity sensing capabilities and understanding how the structure of the fluorophore is related to its function. We demonstrate that due to subtle structural changes, BODIPY-based molecular rotors may become sensitive to temperature and polarity of their environment, as well as to viscosity, and provide a photophysical model explaining the nature of this sensitivity. Our data suggests that a thorough understanding of the photophysics of any new molecular rotor, in environments of different viscosity, temperature and polarity, is a must before moving on to applications in viscosity sensing.

A facile efficient approach to the 2-styrylmalonates via the Lewis acid-catalyzed isomerization of 2-arylcyclopropane-1,1-dicarboxylates has been developed. The efficiency of this method was demonstrated for a representative series of such cyclopropanes. The isomerization proceeds chemo-, regio-and stereoselectively to afford E-styrylmalonates in good yields.

A general method for ring opening of various donor-acceptor cyclopropanes with the azide ion through an SN 2-like reaction has been developed. This highly regioselective and stereospecific process proceeds through nucleophilic attack on the more-substituted C2 atom of a cyclopropane with complete inversion of configuration at this center. Results of DFT calculations support the SN 2 mechanism and demonstrate good qualitative correlation between the relative experimental reactivity of cyclopropanes and the calculated energy barriers. The reaction provides a straightforward approach to a variety of polyfunctional azides in up to 91 % yield. The high synthetic utility of these azides and the possibilities of their involvement in diversity-oriented synthesis were demonstrated by the developed multipath strategy of their transformations into five-, six-, and seven-membered N-heterocycles, as well as complex annulated compounds, including natural products and medicines such as (-)-nicotine and atorvastatin.

The reactions of 2‐aryl‐1,1‐cyclopropane diesters with anthracene derivatives were studied for the first time. Three kinds of reaction products are formed depending on the nature of the aryl group and the substituents in the anthracene. The first one is the product of [4+3] cycloaddition of cyclopropane to a diene moiety of anthracene. The second one also has a seven‐membered ring and is formed by the Lewis acid catalyzed electrophilic attack of cyclopropane onto the C9 atom of anthracene followed by intramolecular attack of the formed arenonium ion on aryl group of the starting cyclopropane. Lastly, the common Friedel–Crafts products are formed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

(3 + 2)-Annulation of donor-acceptor cyclopropanes to alkynes induced by both Lewis and Brønsted acids has been developed. The reaction provides a rapid approach to functionalized indenes displaying intense visible emission (λmax = 430 nm, Φ = 0.28-0.34).

The ability of donor-acceptor cyclopropanes to (3 + 3)-cyclodimerize is disclosed. It has been found that Lewis acid-induced transformations of 2-(hetero)arylcyclopropane-1,1-dicarboxylates containing electron-abundant aromatic substituents led to the construction of six-membered cyclic systems. Depending on the substrate properties and the Lewis acid applied, three types of products can be obtained: (1) 1,4-diarylcyclohexanes, (2) 1-aryl-1,2,3,4-tetrahydronaphthalenes, and (3) 9,10-dihydroanthracenes.

Dienophiles with a difference: 2‐Aryl 1,1‐bis(alkoxycarbonyl) cyclopropanes undergo the title reaction under the catalysis of ytterbium triflate (Yb(OTf)3) in excellent yield (see scheme). Under mild reaction conditions, the major product is the less stable exo isomer. At higher temperatures, the endo isomer is obtained exclusively, as the exo cycloadduct decomposes through cycloreversion.

Ring opening of donor-acceptor cyclopropanes with variousN-nucleophiles provides a simple approach to 1,3-functionalized compounds that are useful building blocks in organic synthesis, especially in assembling various N-heterocycles, including natural products. In this review, ring-opening reactions of donor-acceptor cyclopropanes with amines, amides, hydrazines, N-heterocycles, nitriles, and the azide ion are summarized.