This circuit isn't going to do very much other than clip the positive-going peak to anywhere from 0V to about 3.8V (depending upon the input signal amplitude) and remove the negative-going signal.
D3 & D4 aren't going to do anything - they might as well not be there.
1) D5 clips the positive-going signal to about 5.1V and clamps the negative-going signal to about -0.65V.
2) D1 & D2 pass the positive-going signal through to R2. If the input signal is significantly higher than 5.1V, D5 conducts and clips the signal to 5.1V. If the input signal is only somewhat higher than 5.1V, the voltage divider formed by R1 & R2 reduces the signal.
In other words, the output voltage is about (Vin / 2) - 0.65V for Vin ranging from 0V to about 8.8V.
You can calculate the voltage at which this happens and by how much, taking into account the forward voltage drop introduced by D1 & D2.
3) Because D5 has clamped the negative-going input signal to -0.65V, the series combination of D3 & D4 never conducts. The negative half-going input signal is therefore completely attenuated.